Actually, your estimate of "months" is not far wrong. But let me first take a second to point out that there are in fact two, and arguably three, distinct tchnologies utilized for fusion power production in this universe. The "small" plants used in pinaces and other small craft have a very low (realtive) ceiling on power output, and rely on laser-induced fusion. On a per-unit basis they are less efficient both in terms of generator mass and fuel consumption, limiting small craft to "days" of operation under any kind of operational load; the tradeoff is that the absolute generator size is quite small. The second type or power is the "gravity pinch" fusion generator. These are orders of magnitude more powerful, and several times as efficient. Their minimum size, even with Manticoran levels of technology and miniturization are relatively large, restricting them to vessels of at LEAST 50K-tons (LACs). The third technology is sometimes refered to as "Super-dense" fusion bottles, and is the key component in all of the Ghost Rider cornicopia of weapons and deployable assets. Little detailed information is available on the actual mechanics of these items, but it would stand to reason that since they have not made it into any manned craft they must either be EXTREMELY prohibitively expensive to scale up to starship size or else have the same problem with high maximum power output.
There is some discussion of "cruising endurance" in the series, and the top realistic endurance so far listed would be for Battle Cruisers, on the order of 14 to 18 weeks endurance. The assumption is that Wall of Battle ships (DN and SD) should always be closely accompanied by a fleet train of colliers, replenishment ships, and repair ships. Ligher vessels are either attached as a screen to the Wall or picketing a star system with easy availability of replenishment. When attached as convoy escorts, their endurance need be no longer than their charges, and even the longest hyper voyages take less than 12 weeks generally before a safe port can be found. Only battle cruisers, with their emphasis in Manticoran doctrine, need be able of long, independant cruising for raids on the enemy's fleet train and support infrastructure.
I hope this helps!