Well, I finished a co-op with a league mate a couple weeks ago as Red Bull (no assists). 75% distance is the best really with a short race weekend (that way you don't ruin all your tyres before the race due to the tyre bug)
I am hoping to start another co-op soon but would like it to coincide with our league schedule, that would mean a race every two weeks or so starting from mid June. I'd prefer to compete as a mid-tier team this time though, Sauber or Williams because Red Bull was just too easy against the Legend AI.