Target is caster, I don't think you can cast it on someone else...
A 20th level caster would get 3 minutes from a level 1 spell to have +20 to attack rolls. Seems a bit unbalanced. Maybe a set 3 rounds regardless of level so that it is more usable... it is barely usable even with a bow (Bow of the Guardian has the property 1x/day).
but at level 20 a caster only has a bab of 10, if he has nothing that augment his dex.
And by the time the caster is at level 20, he will be fighting things that has an higher ac than the bab he will have. Think about it, just in the Arnax and the surrounding areas of Miritrix, the deep etc, a lot of the foes there has ac in the +40. The only thing you are doing is bringing the one using True strike, closer to the foes AC. You could always keep the gems and wands of true strike at 9 sec, so it wouldn't be abused by combatant with the ability to use those.
A RL time spell that is based on merely a round and a half (9 sec) is simply not going to work vs a turn based dicing system that with each combatant that is active makes the turns longer.
For comparison purposes, my level 24 monk generally has an AB of about 30
A multi-classed character with 20 caster levels can have a much higher BAB than +10, which is where this spell becomes most useful and powerful. Sorry...are you saying that we should give casters even more ways to solo?
Very few pure casters I know of will go physically toe-to-toe against level-appropriate foes, nor should they really. If we continue down this path, you see how quickly we come up against that old issue, that being the relative power-level of casters at higher levels and their ability to solo where a similar-leveled melee character could not do so without buffs and assistance from said casters...I'm sure you've read the threads on them both.
I'm sorry to hear about your monk having a 30 ab at level 24. But I'm thinking that you either didn't max his dext or str out to have the maximum ab possible, right? There for you made some choices based on RP or on a different kind of build possible. It's a concession you made.
Maybe my example of a 2 minutes and 40 seconds is to high. Making it 3 rounds, well that's18 seconds. It's doubling the time it has right now, but it still is not very useful.
It maybe a one shot one attack kind of spell, but I still think that even if you upped the time, so it stays longer. It will not unbalance the game as it is set right now. In fact, any character that is able to use the gems, will be able to get a +20 boost every 2 turn if they wanted to. One turn to use the gem since you don't have to equip it, just need to have you inventory open, one under the spell. Correct me if I'm wrong but to use a gem of true strike, you don't even need to have Umd. So in between that possibility and raising the time of the spell for casters, what's the difference?