My lilac bush has got quit tall , do I cut it back?

Yes you can cut it back, but I would wait for the fall now

To keep your lilacs looking their best, they need to be pruned, fertilized and shaped almost every year, soon after they are done blooming in the late spring. Don’t wait too long, as next year’s blooms start forming in the summer. You don’t want to cut them off by mistake. Pruning in the winter will remove all the bloom for the following spring.

See if you don't find this informative: http://extension.oregonstate.edu/gardening/prune-lilacs-soon-after-bloom

yes, in the fall. Or spring, but not now.

thank you, can I cut them down quite a bit, there about 10 - 12 feet tall.

Will they bloom better?