An easier way to put it: 1 development upgrade in 10 yearsIn essence, a cooldown period for each upgrade with, perhaps, an Economic idea or related policy that decreases that period.
Yeah, I was just throwing numbers around. I actually like how converting religion / culture / etc. takes time. Would prefer if development of provinces had a cooldown period, like you've said.
The period can be tied to the development modifier, so if you have -20% on development cost, the cooldown time can be lower too. In theory, of course