I went solo in 10hr 40min, all on the Robinson, but I had a lot of fixed-wing time. I know of one student who went solo in just over eight hours, all on the R22. He was a high-time commercial fixed-wing pilot, but even so the CFI was not pleased with the FI who sent him solo.
Robinson recommends a minimum of 20 hours largely to take the pressure off the student. Time-to-solo is a bit of a "hairy-chest competition" for pilots - even the ladies. In particular, the factory quotes one instance of a student being sent solo after seven and a half hours. He crashed and killed himself.
Ironically, I've always found that many of the best pilots I know had real trouble in the early stages of training - came close to giving up, or escaped being washed out by a hair's breadth in the military. There seems to be absolutely no correlation between time-to-solo and eventual competence.