Really not necessary with laser guided weapons, only GPS guided weapons. (Not exacly an answer to your question , but still.)
Yes that's how auto lase works , but under certain circumstances lasing too early can make your bombs miss since they will waste potential energy homing in on the laser while too high in the air. Personally i always use 10 seconds, but anything in that ballpark will do. I've used between 6 and 15, eventually i settled to just stick with 10.