This can vary depending on a number of factors, the main one being how much it is used. This timetable is a rough guide:
Light use (less than 3 hours per week) – every 8 months
Medium use (3-5 hours per week) – every 4 months
Heavy use (more than 5 hours per week) – every 2 months
However, as it can be difficult to judge this, it is a good idea to check the lubrication of your treadmill at least once a month. If in doubt, apply a little oil. You can find instructions for checking and lubricating your treadmill in the Self Maintenance area.