Standard advice is to work out your 1 rep max (the heaviest weight you can lift, so heavy that you can only lift it once), then for power training (sprinting etc) do a set of 6-10 reps at 60-70% of that rep max, with 3-5 minutes rest between sets. Jury seems to be out on how many sets to do, some say there’s no advantage in doing more than one! A lot of people play it safe and say x3. Then let it recover for a couple of days!! (this is as important as doing the initial exercise). And don’t forget to stretch, otherwise your muscle will shorten and you will lose some of your hard fought for gains as well as risk damaging the knee joint.
Good info here:
http://www.brianmac.co.uk/weight.htm