Practical upward limit for skills?

[DethBySquirl]

I'm designing a character for a very high level campaign (level 18) and running into some problems with skills. Namely, at what point I stop investing in one and branch out to others. Is there a practical upward limit for various given skills? Like, once I reach a given bonus is it better to invest in something else? Or should my 2+int skill points character just be REALLY good at two things and relatively terrible at everything else?

[Bob Bob Bob]

Depends on the skill. No such thing as too much perception. Knowledge (for monsters), bluff, intimidate, sense motive, stealth, acrobatics, basically anything with an opposed check (or where the DC scales) you want as much as you feel necessary (which is usually as much as you can get). Everything with a fixed DC (or one specific fixed DC you want to hit) you're basically aiming for a take 10 being enough to succeed (craft and knowledge usually).

But, really, depends on the skills you're talking about. If you have specifics I'm sure we can offer more helpful advice.

[Rub-Eta]

There are multiple factors that play into this. What your character concept is, class you're playing, roles you're expecting to fill and what skills in question.

My advice would probably be to invest one of your two skill ranks per level in Perception and then spread the rest out to pick up class skills bonuses.

[DethBySquirl]

The character I'm building is a high-level aasimar Shaman/Paladin (basically an Oradin variant). The GM is also using the optional Background Skill system, which gives an extra 2 skill points a level that can only be used for Appraise, Craft, Handle Animal, Knowledge (engineering), Knowledge (geography), Knowledge (history), Knowledge (nobility), Linguistics, Perform, Profession, Sleight of Hand, and adds the new skills of Lore and Artistry, all of which are deemed "background skills", with everything else being "adventuring skills".

The adventuring skills I'm primarily looking at are Perception, Diplomacy, Knowledge (Nature), Knowledge (Planes), Heal, and Fly (because I'm getting the aasimar Angel Wings feat). The background skills I'm interested in are Linguistics (using the Truespeaker racial trait to get 2 languages per skill point spent) and two different Performs (string and singing), due to being a paladin of Shelyn.

The problem is a limited number of skill points per level, though 18 levels to distribute them. I'll get 4 per level for the first 4 levels, then 2 (or 3 if I take the favored class bonus for paladin into skill points) afterwards. Which means...44 points to distribute across 6 skills, and another 36 points to distribute across 4 others.

[Rennaivx]

As has been said before, hard to get too much Perception. Opponents get chances to put ranks in Stealth at the same rate, so it helps to do what you can to keep up. Fortunately, the rest of your desired skills have fairly fixed DCs - for example, there's no point in being able to hit a Diplomacy DC higher than 25, since there's no benefit to it (helpful is the highest attitude you can get).

Likewise, the difficulty of Knowledge checks to identify monsters is 10 + the monster's CR. If you had nothing but skill ranks, you would have to put in a rank each level to keep up on average, but if you have a positive INT modifier or either of those as a class skill you can get away with having less ranks and still have a good chance of succeeding. The highest common Heal DC is a 20, and the highest common Fly check is a 25, with your maneuverability possibly helping.

So I'd probably put 18 ranks in Perception, 7 ranks into the Knowledges, and 6 ranks into Diplomacy and Fly. That gives you probably about the best shot at being successful with all of them. Knowing your stat modifiers would be helpful, so we could know if anything needs to be prioritized differently based on that.

(As for background skills, I'd probably just split them up evenly between everything you want. They tend to be more for flavor than anything - even with Linguistics, you'll run out of new languages to take eventually.)