A large company I was working for transfered all money out from Paypal every night. I would never leave money with Paypal or Stripe.
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.
My experience with this is that I sold an iMac on eBay, then a week later PayPal informed me that the person who paid in fact has stolen someone else’s login, and so they sent a debt collection agency after me to make me refund the money, even though I no longer had the iMac.