I was analyzing and archiving some of my old games and I found this forced mate in six (click Move List for alternate lines):
That's odd... My computer seems to have missed that one, but it only adds another move (33. Qh8+ Nf8 34. Qxf8#). So, make this a mate in seven, then.
It's still a boring mate in six if you play Qxg6+ on move 29