Question
Hi Tom:

I have, what I like to refer to as the "Great Wall of Lilacs" running along side of the sidewalk beside my house (I live on a corner lot). I've trimmed them before to encourage new growth, but this year they are becoming a danger to pedestrians as they're hanging over the sidewalk and need to be trimmed badly. But I'm afraid that severe trimming may harm the plants, so when you provide an answer to my question, can you please tell me how far back might be "too far back" and how many seasons I'll have to wait to see blooms again? Thanks!

Answer
Hi Dana,
Thanx for your question.  Here is a very helpful website that tells how to handle your situation.  You may have to do some serious pruning down as far as you deem necessary and then it may take another season (most likely just one) to have the shrub start blooming again.  I hope this helps.
Tom

http://www.helpfulgardener.com/lilacs/03/pruning.html