They've been through at least two winters (winter when they left the farm and winter at the prison)
It's been ~515 days since the outbreak. Season 2 ends in the fall. Season 3 Episode 1 begins in the early spring. Andrea, T-Dog, and Carl each mentioned the winter they just endured.I thought the time that passed was established at 18 months.
Currently, we're about 6-7 months removed from when they moved into the prison. So, there was no winter while they were in the prison.
Source: The Walking Dead Wiki