I prune when I can, when I have a pruner in hand or when I see something I do not like...
Yes the bulk of it I try to do when the wood is dormant and I do not have to wade through deep snow,,,, But my trees are not my full time job and I have too much other stuff going on to even pretend or tell anyone here that thee only time to prune is a 2 week window of prime dormant time in mid winter. Like above mentioned Dec/post deer hunting is a good time as is early march. However, realistically I prune whenever I can find the time and the need is present. In the active growing season I may not make aggressive cuts but I will walk around and nip off smaller branches on younger trees if I see something I dont like and I will do that anytime of the year.
As for percentage 25-40% is the range I have been shooting for. I let too many of my earlier trees go without good pruning and shaping practices and I am paying for it now.
Playing catch up is a bigger pain then just learning to be more aggressive with younger trees and getting them "right" in the beginning... and good luck with figuring out what is right - so many different trains of thought on working your trees. I should have learned how to prune fruit trees long before planting them. The only good advise I can give is do not be afraid to cut on your trees - and there are limits, best times, and better methods but 90% of us do not even come close to pruning our trees enough. That being said also learn non pruning methods like limb spreading and methods to pull down your limbs to have better crotch angles. Those tricks will pay off for as long as your tree is alive.