Idle curiosity had me looking up the lengthiest casting times for a non-ritual spell. A few spells have a 24 hour casting time, such as awaken, hallow, and unhallow.
What does this mean? I think any caster that can pull that off deserves a medal at the very least. To cast a spell, you have to make a minimal concentration check each round. Even if taking ten in perfectly pleasant conditions, you would have to be superhuman to do so past ten hours.
To skip sleeping, eating, or drinking isn't bad for heroic PCs; at least for a mere day. To skip restroom breaks though, that is inhuman. So for all those casters that manage to pull off those full day casting times, congrats! Maybe next time you will wait a few levels and cast wish or miracle instead, that way you can do the same thing in a single round.
Fun fact - scrying takes an hour to cast and only lasts a minute per caster level. Talk about a lot of effort for a brief glimpse. It reminds me of the early internet days when you would take hours to download a picture from an old modem connection.
Any other thoughts?
You don't make a concentration check every round unless something distracts you. That's certainly a possibility if you try such a spell in the field, and you might as well forget it if you're in the town square. Otherwise, it's pretty much auto-success.
As a seasoned practitioner, you should have developed the discipline required to stick it out. If not, there are things the the ring of sustenance, or perhaps the spell's somatic components include ritual ablutions and such, possibly including "cleansing one's temple"? Depends on the spell, I suppose. Suffice it that spellcasting isn't necessarily chanting and gesticulating wildly for the entire spell duration.
...I just got an idea for an "arcane filibuster" abjuration...!
fascinate sounds a lot like "arcane filibuster". You wanna go nuts, check out the Bard Archivist archetype with its 'lamentable belaborment' and 'pedantic lecture' performances. ;)