I think the earliest you could submit it would be to take it at SLS this May/June. If you don't take it there or apply for SLS (and thus don't have a pre-candidate application online), then the actual candidate window to apply for WP directly wasn't until mid-August sometime. My son had taken it at SLS in May, so when he retook it for USAFA in July and got a better score, was able to get it added in then since he already had a candidate number. Without SLS though I don't think you'd have that until at least August.
I'm sure the true deadline isn't until the actual application deadine (January? March? Not sure), but you'd really, really want to have it done as soon as possible to have your application complete. As a minimum before December or so.