How fast should a Triac be switched for a dimming application?
Triacs do not switch 'off' except when the current drops to (almost) zero - a few mA in the case of the one you've linked to. 

So, you're stuck with the mains frequency (times two, since you can turn it 'on' on either the positive or negative half-cycle). You would normally switch the triac on at some point after the zero crossing (a pulse of something like 100usec is sufficient if the load is resistive) and then wait for the next zero crossing. You should keep the timing similar on positive and negative half cycles. Note that it's significantly harder (2-3x the current) to trigger when MT2 is negative and the gate is positive.