Sellect,
There appears to be no way to give an accurate answer to your question, if I were able to predict hours in bases I would bottle and sell it!!
I guess the common sense view is to look at what happens at the base and make an intelligent guess. If the base has a reduction of a/c over the winter you will likely find it overcrewed and hence a lot of standbys. If you are in the UK, the bases seems to have a slightly more seasonally adjusted utilisation of aircraft and this will affect hours.
A year ago F/O's seem content to say they got around 750hrs/yr. Someone above said 600-650hrs. My experience (me and friends around the network) would probably split the difference and say you can pretty much guarantee 700hrs from April 1st-March 31st each year, although the bad months can be really bad (see Nick14's comment).
Wally.