BOAC
Orbital boost are done roughly once a month. This is from NASA:
Even though the space station orbits in what most people on Earth would consider to be the “vacuum of space,” there still are enough atmospheric molecules that contact the surfaces of its large solar array panels, truss structure backbone and pressurized modules to change its speed, or velocity, which is about 17,500 miles, or 28,000 kiliometers an hour. The station is so large (as big as a football field with the end zones included) that the cumulative effect of these tiny particles contacting its surfaces reduces its speed and causes a minute but continuous lowering of its altitude, or height above the Earth.
To fight this tendency, thrusters on the space station or visiting vehicles such as the space shuttle, Progress resupply vehicles or ATVs are fired periodically to “reboost” the station. These reboosts, however, come at the cost of fuel, or propellant, that must be launched from Earth at significant cost.
At its current altitude, the space station uses about 19,000 pounds of propellant a year to maintain a consistent orbit. At the new, slightly higher altitude, the station is expected to expend about 8,000 pounds of propellant a year. And that will translate to a significant amount of food, water, clothing, research instruments and samples, and spare parts that can be flown on the cargo vehicles that will keep the station operational until 2020 and beyond"
O&H