I am making this recipe now and wondering how long will the sorbet & raspberry mixture last in the freezer
The freezer is a time machine, an aide to food production. It doesn't improve anything, it just slows the degradation process. How much deterioration of food quality varies from person to person.
Personally, I think ice creams and sorbets are at their best quality the day they're made. Are they going to be as good a week or month later when you pull them out of the freezer? No, but they might be good enough for your taste buds.
It's really up to you to think about how you feel about fresh ice creams and sorbets and how you think you'd feel if you start noticing product quality degradation. Some people only use freshly ground black pepper, many are happy to use something that was ground in some factory a year ago.
Based on my experience with both homemade ice creams/sorbets as well as commercial versions, I would try to eat the homemade stuff within a week. But that's just me...
It's your call.
I'd think at least a couple of days, up to a week. And since presumably you're adding it to Prosecco, not eating it 'straight', I think you'd be fine. Sounds delicious, btw!