The PW2 is based in the Virginia S9G with a reactor life of 33 years. That gives a bit of leeway over the planned 25 service life of the submarine for up to another 8 years - depending on when the reactor went critical.
Assuming Astute’s reactor went critical in 2007 when launched she should be able to manage until 2040 without refueling. Unknowns are radiation fatigue in pipes, build up of radon in the core and other, well, unknowns.
Problems with the earlier Virginia subs should be able to give advance warning of issues. USS Virginia was launched in 2003, so 4 years ahead of Astute.
Downside is they never built a prototype so the reactor in Virginia is the first of the type and everything about the life is an estimate.