I personally go for nothing less than 40%, more often like 50-60% and it’s all odds, all a chance to level up (and there is no memory, just like a dice has no memory).
I have Maeve’s at ability 10 as maxed 3 stars (ability maxed even before), Hookfang at ability 3 ready to be trained to 2 stars and then there’s Vingthor - she refused to level her ability until 2 stars level 25ish, luckily some yummy 3 and 4 stars made her change her mind (now ability 6 as 3 stars level 27).
What I try to say: Keep in mind that if you go for a 33% chance the odds are 2:1 to not level up. That’s pretty much and I honestly don’t see the point in not leveling abilities so far if your dragons had like 3 feedings?
Even with a 50:50 chance (which means 50% leveling chance aka feeding 3 stars of the same color or dupes) odds are like leveling up every second time if leveling had a memory.