I'd say 200 hours
This is highly subjective and very Dependant on the individual pilot. Some pilots could probably be safe right from their first solo, maybe some others should never have anything faster than a C152.
My opinion is based on two things.
First, the stats I have seen indicate that you are more likely to get killed in the first 200 hours. There's another bad spot getting near the 1500 hour range (where I'm at) caused, they think, by complacency. I'm hoping to live through that one.
Anyway, you're going to make mistakes throughout your flying career, but you'll make more in those first 200 hours. Making mistakes in a slower more docile airplane is less likely to kill you.
In terms of landing speeds and handling my 9A is as easy or easer to fly than a 172. I don't think bad landings is what kills new pilots though.
I think its weather judgment, fuel management, etc, etc.
Second, as one already pointed out, there is speed management, which really means staying ahead of the airplane. I've done most of my flying in Mooney's, so the transition to the 9A was a piece of cake in that regard.
I remember the first time I flew a fast airplane and had to learn the concept of starting a decent long before the airport or even airport area was in view. Its all simple, but was an adjustment. I was glad to make that adjustment with a few hours under my belt.
You've got to be thinking out 30-to-50 miles ahead of the airplane and I think the private pilot training regime just doesn't provide that oportunity.
When you are flying, you have x amount of things to do to stay ahead. Flying at 160kts rather than at 115kts gives you almost 40% more things to do per minute to stay up with the airplane.
Add in a constant speed prop and you've got one more thing to do.
If you find yourself ahead of the airplane and waiting impatiently for the next thing to happen most of the time, then you're probably ready to move up in speed.
If you are closer to just keeping up, Id say wait until it gets a bit boring.
All that said, a bunch of pilots seem to start flying RVs when they have very few hours. I think most of them live - your mileage may vary.