But if you have some time period in mind, here are some tips on choosing your dates.
Winter season
You should be able to have access to skiing resorts of Krasnaya Polyana from mid-end December to early April.
This year January 1 to January 10 are official national holidays in Russia. This means Krasnaya Polyana will be packed with tourists, and prices will be very high on everything, including plane tickets and accommodation. In addition Krasnaya Polyana is a favorite place for our President, Prime Minister, and other high ranking officials to hang out during these holidays, which means parts of skiing resorts will be inaccessible. So, my suggestion - if you planning to in January, come after January 10th.
Long winter holiday scheduler in Russia
2010: January 1 - 10
2011: December 31 - January 9
2012: December 30 - January 8
Summer season
I'd say this mostly depends on how warm you like your water, how many people you'd like to see around, and how deep is your wallet
Personally, I don't like very warm water, and can jump into the water from 18 degrees Celsius. But many people find this chilly. Water usually gets to 18 at mid-June. High season starts in the middle of July and lasts until the end of August — lots of people (many students), warm water, hot weather, highest prices of the year for travel and accommodation. It may be very hard to get train and airplane tickets out of Sochi for the last few days of August - this is the time students get back home (schools starts on September 1st in Russia).
Then comes the autumn season (aka "velvet season"). It starts mid-end of Spetember and lasts while the Nature keeps the Sea warm - usually until mid-end October.
Off-season times
Spring is nice - it's warm, the city is usually cleaned and prepared for the tourists by May, and even though it's too cold to swim for most at this time of year, it very pleasant to be in Sochi during this time.
Fall can be rainy and short.