You can leave your paypal account as long as you want. If your credit card expires, obviously your paypal account is going to be inactive (that is, you cannot purchase anything via paypal) until you have updated your new expiry date numbers. If you have a direct debiting or saving, I'm not really sure as my paypal is link to my credit card.
Your PayPal account is almost like a bank ... you don't need to take money out of your bank every week, and the same is true for PayPal. However, as already stated, if you make any changes to your banking information or credit cards, you will need to make sure that you update that on PayPal, as it can cause a world of trouble for you.
I think you can, but I'm not for sure. I think there is a small charge or something
