In the Campaign my group is running, they have obtained a fortress that I honestly never intended them to be able to get. And yes, I know as a GM I could have denied them the fort, but they really did earn it. My question is what is a fair amount of gold to for them to pay to maintain the fort?
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Another aspect to consider is the non-monetary costs of owning a fortress. Particularly if it's out of their power scale (which you seem to imply that it is), a fortress is going to be a target for all manner of unsavory folk trying to claim it for themselves. Your players should expect to have a small adventure relating to the fortress at least once every six months, more often if the place is especially awesome or in a dangerous area. Not only that, but these adventures won't always come at the best time for the players, and they won't be something that can be put off.
Additionally, your players are now, for better or worse, political agents in whatever realm that the fortress is located in. The rulers of the realm (and any nearby, likely) are going to ask the party for favors in a variety of ways due to their newfound position. They may be asked to shelter refugees from a war zone, contribute levies to the King's army, or collect taxes from nearby villages, all of which can be excellent role-playing experience.
Of course, if the players aren't keen on handling these things, they could always hire an administrator. But this will cost them even more each month. And you never can trust those administrators, always scheming behind your back to take over the real power.
There are two directions you can go here.
However, if this is a good thing your players earned, don't use it as simply a burden, use it as a benefit and especially an opportunity for drama. Let it attract unwanted attention. Surely someone will want to take it from them, there are dangerous monsters nearby, or a group of outlaws or refugees think it's a great place to take up residence. If the players think its something worth having, someone else will too.
The Dungeons and Dragons Rules cyclopedia has a good section on this if you can find a copy. It has flat monthly costs for various retainers and specialists. You'll probably want to adjust things to fit your own campaign setting and rules system but I'll lift some figures directly out of the book
At minimum I would say a fortress would need
A seneschal (4000GP/Month) to manage the fortress in the party's absence
A Castellan (2000GP/Month) to see to the fortress' military needs
A Chief Steward (500GP/Month) to manage the household staff
Servitors such as cooks, armourers, grooms, sheriffs, bailiffs, tax collectors, weavers, gardeners etc. are 5GP/Month Peasants don't require pay - but do require protection (they also pay tax ;) ).
If it were me I'd say roughly 7000GP/Month for a basic functioning fortress with a token guard force (maybe 20-30 men?).
Thats if they want to run the stronghold as a functioning dominion though. If they merely want to occupy the fort and hire some mercenaries to defend it then the costs would be significantly less - Using the rules cyclopedia again
10 light footmen - 20 GP/Month
10 Heavy Foootment - 30 GP/Month
10 Archers - 50 GP/Month
100 GP/Month for a token defence force capable of making a roving band of goblins think twice - maybe another 50GP to add in a couple of cooks, an armourer, and a captain (2nd level fighter) and a bit of petty cash for supplies.
As your party is 2nd level this is probably how I would start them out - as occupiers of the fort. Then ramp up the costs steadily as they advance in power and status and attract people to their banner.
Its a pretty cool situation though - there's loads of scope for some great adventures there.