They typically wont arrange collection unless your RMA is approved (if that's what you're asking)?
Not sure about timeslots for collection, I think you do, or you may just be given a day. If I remember the re-delivery is the same as the original delivery (ie you'll get a 1 hour timeslot).
Your best bet would be to give PCS a call to be sure, but it depends on when you requested the RMA. As far as I know they wont arrange the collection unless they've approved the RMA (makes no sense to do that in my opinion). They will approve the RMA, collect the PC and then test/repair as required before sending it back, I doubt you'll be able to pick a day for re-delivery.