Board Thread:Questions and Answers/@comment-7920645-20130407151331/@comment-5102537-20130407192904

The time gap between season two and three was one year (the year that Morgana went missing), same with season three and four (the year that Arthur was regent king after Morgana's and Morgause's attack).

A time gap between season one and two wasn't mentioned anywhere. So all in all, the show skipped at least five years.