I'm quite sure it's unbounded. People have already found ways to play an unbounded number of other various cards in order to generate an unbounded amount of money and buys on a turn.
http://forum.dominionstrategy.com/index.php?topic=17800.0
Yes, there are many ways to make the number of turns unbounded.
The easiest is probably by using Scepter and Mandarin.
Embellish with a couple of support cards and you have a simple loop.
For instance:
With a Voyage, an Apprentice, and a Graverobber in play; a Mandarin in the trash and a Mandarin in hand with Scepters...
Play Scepter to replay Voyage
Play Scepter to replay Graverobber, gain Mandarin from trash, Scepters return to top of deck.
Play Scepter to replay Apprentice, trash Mandarin, draw 3 Scepters and the other Mandarin.
repeat ...