Exact width of the window may be adjusted case by case and depends on the stuff you sell — e.g. if it's digital goods with immediate delivery it may be shorter, if it's physical goods it may be certain amount of days after delivery verified by tracking number. In my case I was working for a company that provided proxy auction participation support, PayPal forced 30d non-negotiable hold on all our transactions.
So, you may transfer it every night, but it's safe to assume that 25-100% of your monthly revenue going through PayPal is sitting there at all times.