It's best to put the doe in with the buck ONLY to breed. Witness them breeding then remove the doe and place her back into her cage. This will guarantee most of the time a litter. Now, some will miss but it's all a gamble anyway. Good Luck
When I raised Holland Lops, we had a buck that was 7 and still giving me litters. But it really depends on the individual rabbit. I retired him at 8 when his production went down and he just couldnt get the job done anymore.
I have a few Mini Lops and French Lops for sale. Located in Central IL. PM for info Prices are $40 for Mini Lops and $50 for French Lops. All pedigreed