how often to trim?

When you go to a salon, don't ask for "a trim" or "to have the dead stuff / split ends removed". Just ask for exactly the amount you want taken off. If you only want a half inch removed, ask specifically to have a half inch removed. Keep a close eye out and make sure that's all that's getting removed. If they take off much more than that after you were specific about it, you have the right to complain to the manager.

Be sure before you get into the chair that you have a nice person who cares more about making you happy than doing what he/she wants to do to your head. (in other words, try to have a conversation with them first about exactly what you want, and if you don't think you trust them to do just that, tell them you want to think about it or schedule an appointment for later...then you can always cancel over the phone if you decide to).

As for your layers, you may want to watch them and see if you're getting splits in the upper layers, but if you're not, don't cut them. If you do cut them, don't take off more than a half inch. Your hair should get less thin looking on the ends once the layers catch up, but if longer hair is your goal, you may enjoy it more if you don't get it trimmed up to the upper layer all at once. That will have to be your decision. Good luck!

this happens to me all the time, i am in the same situation, only ive just recently gotten it trimmed again, and they trimmed the layers too :( now it looks alot shorter and i hate it...and the layers have to grow out again!

but sorry i dont know what the best solution is