I don't think this is a cut-and-dried situation. Some putts don't take as long to read as others...if you're playing in a tournament and you have a downhill left-to-right breaker on greens rolling 11-11.5, you're probably going to be a little more careful in reading that putt that you would be on, say a 15 footer, straight uphill on slower greens in a round with the guys/girls from work.
That being said, I think that every player should be analyzing their putt from the moment you approach the green complex. Look at the mounding, see where the ground slopes (in the big picture, I mean), imagine how a bucket of water would drain off of the green if someone poured it out standing in the center...Watch other players in your group to see how their putts react around the hole.
All of this gives you the general idea of your putt and how it is going to break. When it comes time for your turn, your final read should really be to determine the line you want to hit the putt on AND how hard you want to hit it.
Honestly, when it comes down to choosing the correct line versus choosing the correct speed--most amateurs would be MUCH better served focusing on speed. Keep your misses inside of 2 feet and you're going to have a lot fewer 3-putts...
And after all of that, to answer the question (LOL
)--I generally take about 10-15 seconds behind the ball once it is my play. A few seconds to get the read, a few seconds to line my stripe up on the line I want and then I'm up and getting into my stance.
Cheers!
- Micah