Board Thread:Questions and Answers/@comment-5773355-20121121214931/@comment-5674726-20121121215531

If we assume that each season lasts a year, and take the one year gaps between Series Two and Series Three and between Series Three and Series Four, into account, The Darkest Hour, Part 1 took place roughly five years after The Dragon's Call.

Series Four spanned the period between Samhain and Beltane, plus Morgana's brief reign, giving us another six months or thereabouts.

Arthur and Guinevere were married at the end of Series Four and just celebrated their fourth anniversary.

I'd say that it's nine to ten years since Merlin first arrived at Camelot.