I have three old lilac shrubs which I would like to rejuvenate by cutting
some of the legs down to ground level. About how long does it take for
new shoots to develop after cutting a leg off? And when is a good time to
do the cutting?

A lot of the wood on these shrubs are old and opened up, but the shrubs
are in pretty decent shape. My concern is that no new shoots will develop
after cutting down to the ground.

Thanks,
Chris Swartz
Zone 5