my neighbors'
lilac is about 20 years old and it has spread out about 4 1/2 feet in diameter.
it might have been a bit wider except for the fact that some of the runners are
growing in the neighbors yard and he keeps cutting them down when he mows his lawn.
i took a few of them for my yard...from the mown over 1/2 they've grown to about 6 inches (in a months time). our weather has been a bit chilly - i expect it to grow more quickly once the truly warm weather (during the day and at night) sets in.
generally, i think they only grow about 2 feet at the most a year...i could be wrong on that tho.