there are many factors to timing...

1. what type of player is the villan?
if he's a lagg he will most likely act quickly with all of his decisions, tagg would be most likely slow and thought out. also a key would be to read the timing per person per hand. do they always take the same amount of time every hand or do they act quickly with the nuts and slowly with a bluff. its a good tell to pick up on.

2. what is the players connection?
pretty easy, is he 100% 99% or is he getting disconnected alot. it will be the difference whether you think hes making a decision or his connection sucks.

3. is he playing one table or playing multi tables?

its pretty important to find this out because even if his connection is good and you think he's taking awhile for an important decision when meanwhile hes playing another hand on a different table and not really paying attention to your hand at all.