Probably not the answer you wanted but 26months.
He never slept in a cot at all. In the end I moved him because I couldn't stand the constant need to be touching me. And he was starting to wake more frequently again. Since moving to his own bed he's realized he doesn't need to be touching me every second of the day and can for the first time ever, just roll over and go to sleep by himself. I think if I'd have tried earlier it would have been a battle, but he was just ready.