Plenty of time! The latest we've booked is 4 days before travel, got a bargain but it can be risky!
Current climate seems to mean less deals as tour operators are taking on less chalets and booking less planes.
We've used SkiWorld and they have been good. When leaving it very late, I've found them worth ringing as they can talk you through options, reserve rooms live and advise on flights. Some rooms were disappearing as we talked and trying to book online got 'denied' at checkout. Advice on the phone was excellent - have a shortlist of resorts and requirements plus some flexibility and they will find something great.
Regional flights fill up quick and you may be in for a longer transfer.
Have also booked last minute with Ski Total and http://www.ifyouski.com/
Another more complicated but more flexible option is DIY - EasyJet to Geneva, hire a car or grab a shuttle bus and find your own hotel. Can give more details if that's of interest...