Drastic pruning of a cherry tree

I have a large cherry tree that needs drastic pruning, It's 35ft tall and 25ft spread,its first branches are 15ft above the ground. The gaps between the branches are so large that  even when in full blossom the impact is minimal. If I top the  tree to 20ft most of the small blossom baring branches will be lost. Will, I get new growth from somwhere if I take this drastic action? When is the best time to prune? 

You're unlikely to get a satisfactory result from what you're suggesting. Cherry trees are large unless grown on a dwarfing rootstock, which yours obviously isn't. Assuming it survives, the tree will just try to grow as tall again.

Far better to remove it completely and replace it with a shorter tree on dwarfing rootstock if that's what you want.