Personally I prefer not to think of that, I'd rather not even see this topic started. It's just like asking 'when will I die?', a bit of a bad thing to think of.
For now, I'm just extremely content that they're reunited again, that they all seem happy and creative, as they always did.
In case it's a prediction what you want, then I suppose they will still be okay to perfrom for the next years, considering they're still in their early 40s. Yoshiki probably will have problems with drumming but still, I think that won't be enough to spare him from the stage.
Let's hope it lasts 'forever', that's just what I wish. I wouldn't like to see them giving it an end like they did 10 years ago.