I wonder at this idea of mothballed space ships being difficult to maintain/recommission. Many of the aspects of mothballed fleets on Earth would be non-existent in space. Especially if you chose a system with a relatively gentle stellar climate. Additionally, if you vented the atmosphere of the ship, I imagine many of the internal components would be more in a stasis than undergoing any sort of decay, avoiding their requirement for maintenance. Sure, temperature can have its own effect on some materials, but, aside from emptying a ship of perishables and toxic materials, then resupplying for active duty, I wonder what components would need heavy duty maintenance.
Seems to me, a mothballed spaceship would require little in the way of maintenance to return to active duty. Even decades later. Perhaps there are factors I'm not considering however.
That being said, I don't think it's difficult to assume that any ship being recommissioned, after decades of being mothballed, would undergo some period of refitting. A few weeks or a few months. To make sure all of the systems are functioning, replacing or upgrading minor components (like displays, lighting, etc.), cleaning dust or debris (a monumental task itself for a large ship), cycling and overhauling atmospheric systems, and, perhaps, replacing or repairing parts of the structure. But, not more than 3 or 4 months, surely.