The Astute PWR Core H reactor has a design life of 30 years, so it will reach the planned out of service date around 2037.
But reactor life isn’t a finite hard cut off - it just means power starts to degrade - HMS Trenchant had a planned reactor life of 25 years and was finally decommissioned after 36 years (1986-1922).
So, assuming the first SSN-A is laid down in 2030, and is launched around 2040, Astute should reasonably be expected to last until she is commissioned.