Keeping Muscat de Rivesaltes
Treat it like normal wine, 15% isn't enough alcohol to preserve it. Refrigerated it will be drinkable for up to 2 weeks and you should be able to cook with it for a bit longer than that. I'm not a huge fan of freezing wine, especially if you're going to be thawing and then re-freezing it, although for cooking you can get away with it. 

Incidentally, most sherries do not have a long shelf-life and should be treated the same way.