It's best to leave it until the flowers have finished (normally second half of September)...and don't cut too far down - leave some of this year's growth. If you cut back to the woodier, older growth....it won't re-grow.
I may be wrong but we wait until it dies back and dries out .. then it gets cut straight off about 2 " from the ground. It seems to grow back every year.
Grannie39, You might like to look at the RHS site for pruning lavender, they have a good video on there. sorry I can't supply a link for it.
Good luck.