Only for a couple of days in the fridge. You can freeze it but it reduces the pectin to some extent. Alternatively bottle it for storage - take a sterilized kilner jar and put into a pot of boiling water with a trivet underneath (or the heat will crack the jar). Pour the stock into the jar and boil for 5 minutes before sealing airtight. I've not tried this but I can't see why you couldn't just freeze the quince and then bring them out when you're ready to make some more pectin stock (you'll probably have to allow an extra 10% quince to stock to make up for the loss of pectin in freezing).