Re: When do I know that it's time to adjust the truss rod?
Don't worry about it. You usually only turn it in small increments, then re-check.
The only way you'll do any damage is if you have no clue about what you're doing and you keep tightening it till it's almost difficult to go further, then something snaps.
By the time you're at that point the neck will be severely back-bowed and you'll likely notice that you're killing the neck! Truss rod adjustments are really simple as long as you do everything gently.