It depends on how long your growing season is, how healthy your tree is, and what type of pruning you're doing (severe pruning or detail pruning). Generally anywhere from twice a year for very short seasons to 4 times for the long seasons. Here in Texas I can usually get 3-4 prunes per year out of my elms.
The important thing is waiting for the new growth to harden off so the tree can recoup its spent energy before pruning again. If you prune again too quickly, the tree spends energy on new growth which you then cut off. Then it uses more energy to replace what was pruned and you cut that off too. It may not die in a year or two or three depending on how strong it was before, but over time it'll use up it's "reserves" and may suddenly give up altogether