Shock and awe – I’m still working on Crossworlds, albeit slowly due to school and other requirements.
The plot of Crossworlds involves a pair of destroyers operating on a specific mission of limited duration, so endurance doesn’t come into play, but ships in the ‘verse do have certain limitations in terms of movement and flight.
The majority of a starship’s noncombat flight time is coasting – the crew puts the ship on its proper heading, gets it to its proper speed, and then turns off the engine and lets Newtonian physics take their course. Because of this, the major load on a starship’s systems at slower-than-light velocity is what’s called “hotel” – life support and gravity generation. And a ship’s endurance on hotel is limited primarily by life support consumables, because hotel can easily be maintained using only the fission plant, which has an unrefueled lifetime of 10 years (so technically a starship on hotel-only would be able to last for a decade if it had enough food before needing to be refueled, which as in the real world is a detailed process of deconstructing and reconstructing certain designed-in soft patches in the hull to remove radioactive waste for reprocessing and return fresh fuel cells).
When a starship is accelerating or traveling faster-than-light, there is a certain amount of energy that needs to be used to maintain the Alcubierre drive field (colloquially referred to as a “warp field” or “drive field”). This energy is enough that one or more of the main fusion reactors needs to be used to supply it, along with the energy to maintain navigational shielding in the case of a sublight warp. At faster than light speeds, and this is a predicted effect of the real-world Alcubierre FTL model, a ship at FTL speeds “sweeps up” energy and debris into an area where space is moving at velocity zero relative to the ship itself, meaning you no more have to worry about the FTL asteroid that you’ve just sucked into your warp field. Whoever’s on the other end, however, will have to deal with a bombardment of radiation and debris when your warp field collapses at the end of the journey, a reason why FTL drive navigators are highly paid.
In both of these cases, you’re using up hydrogen to fuel the fusion reactor, creating huge quantities of radioactive Helium 6 in the process (if you are a crew member and wish to live a long and healthy life, don’t hook up a rubber hose to the fusion reactor waste product tank and use it to talk in funny voices, that’s a good way to give yourself all sorts of fun respiratory cancers).
From a writing point of view, the part of all this that really matters, however, is the length of time a ship can cruise without having to tank up. And the answer, in all cases, is “a few months.”
Destroyers, which are patrol vessels among their many, many other qualities, have an endurance at FTL velocities or under normal acceleration of approximately 180 days (six months, give or take), and frigates, which as fleet vessels are primarily tasked as light consorts for the destroyers, have the same endurance. A destroyer tender carries enough consumables to completely refuel and rearm four destroyers and up to eight frigates (a destroyer squadron and a frigate squadron).
Light cruisers have the same cruising statistics as a destroyer, since during their most recent period of military fashionability, they operated as lead ships for a division of destroyers (a full destroyer squadron, at that time, was a pair of light cruisers each leading two destroyers and four frigates for a total of fourteen ships in the overall squadron).
Capital ships – heavy cruisers and battleships – operate independently for shorter times because they have to run very high-energy systems. A capital ship has 120 days’ (four months’) endurance in FTL. They also have a higher amount of redundancy in their power systems than a light cruiser, destroyer, or frigate, and more durability under combat damage (a good thing because their weapons hand out a LOT more damage!).
Corvettes, like Veronica’s Dog Two-oh-Seven in Independent Flight, are typically powered exclusively by a fission pile. They’re limited pretty much by the consumables they carry (about two to four weeks’ worth of 3L ration packs) and their air regeneration systems (about a month or two).
All of this goes out the window during combat, though. During combat, ships typically maneuver under power at all times and additionally suffer damage ranging from mild to severe, resulting in anything from loss of consumables (often atmosphere and food stowage) and weaponry (both from firing and from damage to magazines and/or the weapons themselves) to the loss of significant amounts of fuel bunkerage (fuel bunkers, as in our world, serve as a form of additional armor because it’s far easier to lose a few hundred cubic meters of pressurized hydrogen than it is to lose a few hundred cubic meters of ship systems). Generally speaking, a starship in battle outside of a wartime scenario is going to heave to for damage control and will probably return to base for resupply and repairs.
In peacetime, a starship will often operate like modern submarines, with two crews assigned to a single ship. Typically the “blue” crew will deploy with the ship for the first leg of its cruise, which will typically last 200 days and go through the first refueling; the gold crew will board and run the second 200 days of the ship’s deployment, including its second refueling (and third refueling for capital ships). Both crews will be shoreside for a 100 day period during which the ship is thoroughly cleaned and undergoes a standard refit, and then both crews will train and work up for 100 days. This is LOOSELY based on modern submarine warfare doctrine and has some modifications because this is a space opera setting, not a strictly-speaking military science fiction one.