You could probably go pro on times way slower than those listed above. All you need is a few sponsors, a good homepage and maybe doing some kind of promotion for your sponsors besides just having their logo on your shirt. Look at all triathletes , surfers etc thats nowhere near worldclass but still gets enough money to go by on it.
Still, you got to have SOME ability but I think this would be enough:
800: 1.50
1500: 3.45
5000: 14.10
10000: 29.30
marathon: 2.28
When you're in that range it's more about how well you can represent the brand/product and possibly your apperance has quite a bit of market value too. Just look at Anna Kournikova in tennis, really hot and makes lots of cash but not an outstanding tennis player...