Between the end of season 5 and the beginig of season 6.
If you watch 6x03 "Epic Fail", you can see Cuddy acting weird, specially when she goes to visit House. And House is cooking, Cuddy is about to tell him something but she is interrupted by the chinesse woman.
Right there, Cuddy had doubts about her relationship with both Lucas (probably she is dating him casualy) and House, but after House tells her "She is not the reason he is leaving" and there nothing to discuss. Cuddy says goodbye to him, with a smile. She is in peace about them

Later in the episode Wilson says Cuddy was "busy" for their weekly lunch. (Maybe in a date with Lucas?)

Then House comes back to work and Cuddy's attitude changes.Because now she is commited to being with Lucas.

By Known Uknowns, Cuddy and Lucas had been together for at least 3 to 5 months.

interesting timeline, i think is about right