Quiet time needed for initializing purchasing cubes

When initializing my purchasing cubes and filling the setup tables ...

How much quiet time do I really need on the R/3 side? Is it just the amount of time it takes for the setup talbes to be populated or should I include the time it takes to actually load the data into BW?

Also, is there a way to reload just a few documents even though this is a delta load?

Thanks!

Alina