Go to this page:
http://www.nivlam.com/euro/Put in your current masteries, and see if you can figure out what you're going to need to put in to get the maximum masteries @ 80. Hit Calculate. That's the total number of SP you'll need.
Depends on you, when you want to farm. But you're probably going to want to. You don't gain enough SP
without a gap to make your char strong enough to last a second @ level 80 in a PvP.