There are lots of recommendations to do the reps slowly - like with 2 seconds + 2 seconds, also some machines have rep timers of 2 seconds + 4 seconds. But if you are targeting fast twitch muscle for strength and bulk, why wouldn't you do the reps quickly to fatigue the fast twitch muscle fibers?
You do want to be doing the reps as quickly as possible, but if as quickly as possible is half a second for the positive and negative phases, you need to be lifting heavier weights. I wouldn't worry too much about the exact time, but if you're lifting heavy weights that will promote strength gain, you probably are looking at around 2 seconds up and down.
Also, fast twitch is a bit of a misnomer. It's better to think of the fibers in terms of slow recovery and fast recovery, because there at least the way they're described accurately reflects how they behave. The 'fast twitch' muscle fibers are the ones most important for strength, but you're not necessarily 'twitching in a speedy fashion'.