At what speed could i open or close my convertible top on a Porsche Boxster S

You should be parked when you do that!

Attempting to put the top up or down even at a slow speed can and will do damage due to twisting from wind.This can cause problems when attempting to put the top down and it not aligning up with hold downs.

standard procedure is to do it at a stop for all cars.

some hard-top convertables can be done while driving l think, but its always preferable to come to a stop.