I try to notice betting patterns and read into them, for example, long pauses meaning it's a tough decision, etc. But really, how reliable can this be, a long pause could be the result of someone on a bad internet connection or someone that just isn't paying attention, only to look up and find the nuts, or someone that is playing on multiple tables and isn't much of a multi tasker.
As for how long you should take, you should take as much time as you need/want. It is courteous to act quickly, but if I see someone hit the zzzzzzzzz when someone takes 3 seconds to make a move, I will sometimes start taking longer too, so that I can put them on tilt.
Also, if someone bets big and you take a couple seconds to make your decision and they hit you with a zzzz or something that could be a tell, they are either eager for you to call or to fold (you just gotta figure out which one )