Role playing an alignment shift and how long should it take

[McDaygo]

Say I have a story based player that is a CE cleric but due to some encounters out of game told me they saw growth/rp potential in redemption to eventually be converted to CG (still a cleric only things would change are deity, domain and channel Ability from negative to positive). Clearly this won’t be an over night change.

My question is how long should this take in game via another cleric converting them to the faith? I also assume they will spend some time CN on the journey to a new faith?

I found these rules for Converting NPC:

Cohorts and their masters often share alignment, but a common religion can be just as important. A cohort may share a leader’s faith when first recruited, or a PC may seek to convert a cohort as their relationship develops.

While PCs may choose to change alignment, such changes for NPCs may require magical assistance (see the atonement spell). Alternatively, you can proselytize for your patron deity, using your powers of persuasion and social inf luence to indoctrinate an NPC in the dogma of the faith while inspiring their fealty.

Converting an NPC to your faith is similar to modifying their reaction with Diplomacy. You can only attempt to convert NPCs who are at least indifferent toward you, though you can take time to make a hostile or unfriendly NPC indifferent, and then begin the process of conversion. The DC of a conversion depends on how many conversion steps away from your faith an NPC is, starting at one conversion step for an NPC with no signif icant faith who matches your faith’s alignment. For each step the NPC’s alignment differs from the alignment of your faith, add one conversion step. If the NPC already holds a signif icant religious belief, add an additional conversion step. Note that servants of other faiths— including divine spellcasters who worship a deity and agents in the employ of a formal religion—normally can never be converted to a new faith. The GM has final discretion over whether an NPC will ever convert to your beliefs.

If the GM rules that an NPC can be converted, the DC to convert that NPC is 10 + the NPC’s Hit Dice + the NPC’s Wisdom or Charisma modifier (whichever is higher) + 5 per conversion step. For example, a 5th level warrior with a Wisdom of 12 and no existing significant faith and who is the same alignment as your deity has a conversion DC of 21. Each conversion attempt requires at least an hour of proselytizing, and you can attempt no more than one conversion check per week. For your conversion attempt, roll a Diplomacy check and a Knowledge (religion) check at the DC determined above. If you succeed at both checks, decrease the number of conversion steps for that NPC by one. If you fail either check, you reinforce the NPC’s existing faith and add one to the number of conversion steps for that NPC. If you fail three checks over the course of a conversion effort, the NPC rejects your faith and you cannot attempt another conversion for a year. However, if the number of conversion steps ever reaches 0, you convince the NPC to adopt your religion. Note that this does not necessarily change the NPC’s alignment, so you could manage to convince a chaotic evil harpy to worship her own very odd form of Shelyn.

While proselytizing is a reasonable activity for a religious adventurer, it should be handled with sensitivity, both in character and at the gaming table. Having a PC repeatedly attempt to convert NPCs who aren’t interested is likely to result in worsening their attitude toward the character, and significant effort may be required to restore those relationships. Some players may be uncomfortable with too much time being spent on religious talk, in which case a GM is free to simply treat the conversion as a skill check, without actually roleplaying the talks that allow the conversion check to take place.

So if I understood this correctly because CE is two steps away from CG; it would take at least two weeks of attempts just to change religions. That is fairly clear I’m curious how long it should take for actual alignment shifts.

Again this is the players request but they want some realism for the grown not a push button forced change of alignment

[Bjørn Røyrvik]

It takes as long as makes sense for the character. Ignore mechanics, think about what inspires the change of heart. One of the most famous literary examples (in English, at least) is Ebeneezer Scrooge, who went from basically LE to LG overnight due to a bad dream.
It's perfectly possible to imagine some low-level person, either experience level or evil level, who witnesses a Good act or person so inspiring that they immediately reject their previous ways and attempt to be a new and better person. There may be some hiccups along the way but the general change can be swift.
On the other end of the spectrum you can have a very gradual change, one where the character starts questioning their life choices and beliefs, slowly coming to the conclusion that Evil is not desirable, for whatever reasons, and over time, possibly years, they become Good.
Think about what inspires the change and how interaction with other characters will help or hinder this.

examples:

In my games we've had two instances of alignment change. The first was Grund, a throw-away NPC from an adventure some time ago. Grund was a CE ogre and as nasty as they come. For unclear reasons the players took a liking to him and started wondering aloud if it was possible to convert him, then started fantasizing about making him a paladin. I shrugged and said it would either take a lot of roleplaying or a PC could take Leadership and choose him as a cohort and the process would happen automatically. The players decided not to, but a few weeks later one player changed her mind. In game we had this change of alignment occur off-screen and downtime as the ogre who had known only brutality struggled with the memory of the elf who had not only saved his life, but healed his wounds and, most importantly, smiled at him and wished him well. Grund was on his own and had to figure things out by himself. He had a couple long years of fumbling his way towards a new moral code and trying to live in human society, but eventually he became a paladin (worshipping the PC), sought out the PC and offered his services. Had the PCs stuck around from the start the change would have gone much faster.

The second is a PC who started CN but early on the player said he wanted to become LG, and wanted a gradual change. This was fine, and over the years the PC went from a wholly self-absorbed spoiled brat to a responsible and dutiful ruler. But she's stagnated at LN because either the PC or the player doesn't really understand Good in D&D, and has done a lot of things that proves that the PC isn't Good.