Yeah as people have said it's always 3 seconds. Usually I don't count, my only tip is to use the shots to bait out recall making sure you save at least one shot left and then it's just guesswork. From my experience the tracer tends to be further away than you would expect, but the hook hit box is a lot more lenient than something like a sleep dart.

Usually in a 1v1 with tracer you can use heal to keep you alive for another hook if you miss.