I can see Katy making it to her 4 or 5th album before people start losing interest. Beyonce could easily make it that far(solo) if she wants to. I don't see Adele going past album #4 tbh. I have a feeling she may pull a Dido.
Her other singles weren't that good. Blah Blah Blah ruined a lot of the momentum Tik Tok created. If they would have waited a month or two and released Animal or Your Love Is My Drug as the 2nd single instead, I think the album sales would be a lot better. Plus, it's still better than Rihanna's 1st album and Beyonce's if you count the albums from Destiny's Child as Beyonce's.
We have to wait to see how good/bad her next album will do, in order to make an accurate prediction.
All Of Destinys Child albums have sold more than 2 million worldwide.
It's really not that hard to make it to 6 or 10. I mean look at Cyndi Lauper, she never was able to reach the height she did on her 1st and 2nd records, but that hasn't stopped her from having over 11 studio albums.
Some people find not being able to match previous heights discouraging and therefore stop recording, like Paula Abdul, but others genuinely love making music and will continue to do so if given the chance, like Cyndi Lauper. I don't see people like Katy, Adele, or GaGa stopping simply because they aren't popular.