You shouldn't be given the clearance until it's safe, or at least will be safe when you get airborne.
That's what they've always told me. ATC will apply the necessary wait time and will give you your take-off clearance only when it's safe to go.
However, in practice, that's not how it works at my home base. As soon as the 737 is no longer in contact with the runway, we get our take-off clearance, with the addition 'regarding the wake turbulence of the 737 that just departed'. So wake turbulence separation is left to our own discretion.
And to be honest, I like that. There are regular occasions where I'm able to lift off sooner, and establish an initial angle of climb that keeps me well above the 737s take-off path. And if I make an early turn, there's no chance for me to get caught in his wake turbulence. So in those situations it wouldn't make sense to wait, say, three minutes.