Hard to say because we don't know the current assembly schedule. But based on the wing shipments I expected FAL start last month, but it probably takes more time to prepare FAL start because it's the first A350 with a cabin. Rumors said FAL to start in the beginning of June, I guess I was a bit too optimistic.
Final assembly goes faster with every new frame, MSN3 for example is already having a 6 weeks lead time advantage over MSN1. Airbus already installed and tested a fully equipped cabin on MSN5011 (the real-size cabin mockup), cabin installation will be done parallel to the fuselage join-up and system tests in station 40 and 30, so it should fly later this year.