In my experience, if your queue runs for too long (having a waiting time of 4+months) people tend to forget they had a slot reserved or just use the money they had for it in other things, so I don't reccommend you take pre-orders too long in advance unless you also charge in advance! ;m;
The good side is the people that REALLY want your art tend to stick with it and you tend to have at least a few commissions lined up for coming months :3