In the Season 2 finale during the Kraang invasion, Casey does say this is, "Just as bad as the invasion from last year," meaning the Season 1 invasion. That basically says 1 year past between the end of Seasons 1 and 2.
Then again Mikey still says he's 15 years old to Napoleon Bonafrog in the Northampton arc afterward...but considering it's Mikey maybe he was too dumb to realize he aged a year.
I'm not sure if 5 years passed in-universe like in real life, but it was probably at least 3 years.