Quote from Asterix on the 9th of October 2016:
You just have to have an active premium subscription, which for 3 months is $29.95.
I believe this CSGO season does just scrape in just under 2 months and you could buy it monthly and just save if you sign up at the very last second, but in general across the site most leagues will be 10-12 week.
Next year our plans are for most games across the site to have as little down-time between seasons as possible to fit in with the new model. With each season starting almost immediately after the previous seasons grand final etc.