How many expansions do you think you'll be making after Nocturne? Is there a set number of expansions you think Dominion can have before every aspect of the game's been fully explored and developed, or is it more a matter of continuing to make sets for as long as you have the ideas for them?
My plan is to slow the pace down after the next expansion. How many you can make depends on how complex you're willing to make them, and how little you require them to evoke the basic experience of the game. If you are loose enough on those issues you can keep going for a long time; if you demand simplicity and a classic Dominion feel, there may not be much more to do.
For me though the big thing is, well it's the war between "time spent on Dominion is time not spent on other games, and I'd like to make other games too" and "I'm not getting anything done on other games and like to feel satisfied with accomplishing things; maybe another Dominion expansion?"
So, how many more expansions, it's really down to how well I do on other projects, which is just so hard to predict.