Disclaimer: all of the following represents my personal view.
Craig has given pretty detailed descriptions in the secretary’s report of the day-to-day tasks that land on the board’s table. Have a look at the board meeting minutes. Here are the link to just the last three months:
They give you a good idea about the daily work.
As a board member, you should be able to put aside 3-4 hours a week to just follow the board discussions, participate in important decisions and help with some of the daily tasks. Then there are two board meetings a month for which you need to be able to block a regular slot in your calendar. Because the board is very diverse in terms of time zones, the meetings are most likely to take place around noon-4pm UTC.
I would also strongly recommend that you lightly follow this forum to have an idea what is going in the community. No need to read every tagging discussion in detail but at least skim the new topics and follow the ones more closely that concern the OSMF. Add another 1-2 hours a week.
On top of that, you are free to donate as much time as you wish to board work. There is certainly enough to do. Have a look at the strategic plan to get an idea about the “big” tasks, you could take a lead on. Personally, I’d say the move to the EU is still on top of the list. I haven’t been able to get this as far as I would have wished, so somebody needs to take over this project. There is also still lots to do in the areas of funding and community engagement. Somebody with experience in bookkeeping would be an enormous help. If you are good at motivating people, then getting more people engaged in the working groups is a great way to help out. That’s just some ideas. I’m sure that others in this forum can immediately tell you about other priorities. 
My take on board work is that it is much less about implementing specific ideas. It is more about providing the right environment that these goals can be implemented by others. It’s about bringing together the people with the same idea, about making sure they have the funding, information and tools they need, about having an open ear and provide support when they are stuck and about just leaving them alone when things go well. You should like talking and listening to people because that’s what you will be doing a lot.