I don't generally do anything on new cosplays for a convention (or any event really) until 3 weeks prior to the event. At the very earliest. Except maybe cutting out patterns and pre-washing fabric. I can't get motivated without the panic factor coming into play, and that doesn't usually kick in until two weeks to go.
This year, it's slightly better, because I (a) only have one cosplay to make from start to finish (a somewhat complicated one, but nothing I can't manage) and (b) I'm doing steady costume work because of my schedule:
This past weekend - Sewing work day with my friends
This coming weekend - Cosplay event
August 23rd - AE; basics of costume must be completed for a photoshoot
August 30th - Kumori Con; any details not completed by the previous weekend can be finished in the time between cons
Okay, so I'm actually working on three other costumes (two for friends) at the same time, but they have to be done by this Sunday. Not for Kumori Con, though I'll probably wear mine if I'm satisfied with it. Does that make sense? Aw, they're seifuku, way easier than Ashitaka and all his little bits and pieces.
It helps that I'm currently between jobs, so my time's pretty open. ^^' The one part-time gig I have only takes up one day on the weekend every other week.
The way I like to look at it is, as long as you're not working at the con itself, you're doing fine. I've had problems in the past, which is why I've told my friends "If it's not done, it's not coming." Also, sewing machine does not go in luggage this time. Hem tape and safety pins will cover all faults.