The RN, in the 1960s, wanted 5 boats to have 2 at sea. With 4 boats the had 1 always at sea and 2 for 70% of the time. That was when boats needed refueling about every 3 years. With lifetime cores and the same pressure as the was there at the height of the cold war I would expect that 4 boats could lead to 2 always at sea. (ex RN submarine engineer officer 1972-1984)
By design, never. One was refueled because of a core design concern.
The Ohio class get refueled once during their careers, around the 20 year mark. If you want to keep it in service longer, you'll have to refuel it again. IIRC, the Virginia class and upcoming Columbia class are designed with lifetime cores and won't be refueled during their time in service baring a fault in the design or a need to extend their service lives.
I'm at work, so going off of memory, but I think it's 35-45 years depending on class
I don't think the information is public, but I remember reading open source estimates that four of the fourteen Ohio's were on patrol station at any given time. Of the rest, 1-2 would be coming from or proceeding to a patrol station, two would generally be in some kind of overhaul (for New START this qualifies as 'undeployed'). So probably 5-6 in the water at any given moment, two in heavy maintenance, and the balance tied up at dock. This is someone's public source guess, though since they are usually visible in doc by satellite and there are only two boomer bases, it probably isn't far off.
air-cosmos.com
