Mungry,
As I understand it the bank, like any other new owner, becomes liable for paying current monthly fees as soon as they become the owner (which I believe in law is when the title is changed at the land registry). They are also liable for any fees debts on the property, again like any other new owner, for the current year and the previous year.
This is why of course when buying a property any prospective owner should check that there are no outstanding debts attached, if the property is on a development run by a community the inquiry can be made to the community administrator.
In the case of properties accepted as dacion en pago I believe the bank is liable for the whole debt and most banks insist that fees are paid up to date before they will agree this, for some reason our bank has a few properties taken back under dacion en pago with fees debts outstanding, and which are in debt for fees right back to November 2010.
In our case the bank is already advertising 6 properties for sale on their website. I have written to ask how they can sell these properties a) with debts attached and b) on a development which cannot even provide basic services because of fees debts which of course include the bank's. Their debt currently stands at €52000 which is no joke when we are struggling to pay for basic services such as civil liability insurance and communal electricity. The reply I received, last October, insisted that they were 'working towards making payment' and that 'their interests are the same as ours', which has a slightly hollow ring right now.