[plt-scheme] pause or wait?

When running recursive procedures that have an effect on the screen
(such as an animation) it sometimes nice to have them pause between
each call. I looked for such a PAUSE or WAIT primitive but couldn't
find one. Any pointers?

If the answer is "build one," I'd appreciate any suggestions for good
metrics to correlate with time (such as milliseconds). In other words,
if the solution is something like:

(define (pause milliseconds)
   (expt milliseconds 1000))

Does anyone have suggestions for appropriate calculation?

thanks,

k.