When is the best time to cut back a Laurel.

Depends whether you're talking about Spotted laurel (Aucuba) or cherry laurel (Prunus laurocerasus, plain green, large leaves, woody). If the latter, you can cut back any time, though it is best done after flowering at end of May or in June. Aucuba only has one variety that gets that tall, and that's its maximum height, so I'll assume you have the other one.

I'm sorry, I haven't answered your question properly - in terms of how far you can cut back, I wouldn't take too much off now - trim it back a bit, then give it a good hard prune next Spring. If you're not fussed about the flowers, then do it in April/May. Again, this assumes you've got cherry laurel, not spotted.