panic!
in seriousness, taking a week off every month sounds good. i take a week off posting at the end of each chapter / roughly every 3 months - and if im feeling nervous about deadlines, i extend that to 2-3 weeks. never more than 3.
though thats a cure more than a prevention - prevention is about time management!! i use this spreadsheet by jason brubaker to track my progress:
i set the deadline for each chapter at the quiet week at the end of the previous, and then... hit that deadline. i set internal deadlines; i work on twaw on the weekends, so each weekend has an assigned goal, eg: all blocking, batch #1 thumbnails, etc 2 3 4, inking, colours. though thats my timeplan now that i can bang out an entire chapter before the previous one ends.
when i was a bit slower, id thumbnail and sketch 6 pages in one weekend, then ink and colour and schedule them the next weekend, so id always be adding 6 pages to the buffer for every 4 spent. id still aim to get it all done before page 1's release, but i then had 2-3 weeks wiggle room.