ours is ~45 degrees (ramp with rungs every 6") and our 6-7 week olds struggle with it. i had to put some ridged rubber flooring in between the rungs yesterday. seems better but they're still not really "getting it".
personally i'd say no more than 25-30 degrees if it's possible. (45 is normally regarded as a maximum afaict)
 10 seconds later, just looked outside and saw one of them make it all the way up. they grow up so fast! *sob*