Depends on which "plan" you are talking about. There have been several totally different plans publicly stated by UAL over the past several years not even counting the various internal back-of-the-napkin initiatives.

But officially: when the A350-900 was originally ordered in 2009 (and announced as the 747 "replacement") the deliveries were scheduled for 2016-19. With conversion of the order to the A350-1000 in 2013 the first deliveries were pushed back to 2018.

That said, the fleet plan is obviously fluid and with lots of 787s coming online UAL could either use them for international growth or place them on routes that free up larger 777s for the 747 routes or some combination. The long range planning committee meets twice a day so really the only certainty is the "plan" will change again.