I placed mine about 4 weeks in advance but that was during August.
You can place it now and always add to it. They will send you a confirmation through email.
I also placed mine 4 weeks in advance. Also for our August trip.
I have used them twice and they are great. This one trip I almost did not go over the list because everything seemed to be there. We did have one item missing but it was delivered the next morning.
I waited until a week prior to my trip a few weeks ago and was lucky enough to find that they had 3 appt times left for the day I wanted delivery, but not the time that I wanted. It didn't matter because the resort stored my perishables in their fridge until I had them brought to my room.