Around the time when he first guest starred on Gossip Girl (season one).

Well,they started dating in 2007,when Sebastian guest-starred on Gossip Girl as Carter Baizen in season 1. Then in 2008 they became more and more open about their relationship. They started going out on premieres,cinemas,parties,concerts and etc.
Even in summer 2009 ,they even stated,that they will share a flat together and even are engaged