As foot ^^^ just stated, refunds can be pretty instant so long as they are issued direct from the seller and he/she has sufficient funds in there paypal account. If the funds need to come from a linked account it may take upto 3 days or so.
IF you have put in a paypal dispute you will not get anything rolling untill it has been escalated to a claim (there is usually a minimum time before you can do this but I may be wrong). Once a claim has gone in it takes paypal upto 21 days to investigate and decide on an outcome (usually on the side of the buyer if its a clean cut case).
One thing to note is that as soon as a dispute or claim has gone in, paypal puts a freeze on the sellers account to the tune of however much is owed. This is then released or paid over at the end of the 21 days. If however the seller has no funds or a back up source linked to there account then you get ZIP diddly squatt unless its under the buyer protection scheme. (see paypoo site for details).
(thats to the best of my knowledge but not guaranteed 100% accurate as my brain is frazzled today
)
Hope that helps
-ChEf