This is a difficult question to answer. There's no single schedule that's adhered to for months at a time. As far as I can tell, every day has a unique schedule assigned to it, based on the amount of crew on board, the experiments that need to be run and the station operational details (cargo vessels docked or not, reboosts, repairs necessary etc.).
You can review these daily schedules (called the timeline) here. There's also a live timeline where you can see what astronauts are doing at the moment.
According to Robert Frost (he trained ISS astronauts to operate the motion control system):
The seven day week consists of five and half days schedule for working nominal tasks and a contiguous 1.5 days off. That doesn't necessarily mean the crew do no work during that 1.5 days, just that they aren't scheduled for tasks other than mandatory things like exercise.
A 24 hour day is composed of 8.5 hours allotted for sleep. 6.5 hours allotted to scheduled work tasks, 2.5 hours schedule for required exercise, 1 hour scheduled for lunch. There is also time allotted for daily planning conferences with the ground, work preparation time (time to read procedures and gather tools), and plan familiarization time (time to review the day's schedule). And then pre-sleep and post-sleep time during which hygiene activities, dinner, and breakfast are completed.
The daily schedule looks something like this:
06.00 wake up, breakfast
07.30 planning conference
08.15 start of the work day: science and maintenance activities plus workouts.
14.00 resume work
18.15 work preparation for the next day, another planning conference
19.30 dinner, then free time
So work from 7.30 to 13.00 is 5.5 h, 14.00 to 19.30 is another 5.5 h, for a total of 11 h including 2.5 h of exercise and 1.5 h for planning conferences and preparation.
In the 2003 report 'Factors Affecting the Utilization of the International Space Station for research in the biological and physical sciences' (download available here), NASA estimates that 2.5 persons are needed for maintenance, on average. I.e. when 3 people are aboard the station, they spend 100 hours/wk on maintenance and get to do science for no more than 20 hours/week. When the crew is larger, science time goes up.
In 2011, the goal was 40h/wk of science time, but it still varied wildly, with 9h/wk when only 3 crew were aboard, and a maximum of 70h/wk.
This article contains an interesting graph of planned science time vs. accomplished time, but it's reproduced too small to be readable:
In the 2006 study "Applying Analysis of International Space Station Crew-Time Utilization to Mission Design", it was found that crewmembers spent an average of 1.9 h per work day and 1.8h per rest day on station maintenance. This doesn't add up to the figures I listed earlier, unfortunately I don't have access to the entire study.
Edit: I recently had the chance to ask André Kuipers (who spent 6 months on the ISS during Expedition 30/31) how much time he was able to spend on science. His estimate was 40%.