Signs such as difficulty in closing, the sash touching the frame, more noise during movement, a tilt function that no longer works properly, or wear in the gaskets are worth having checked early.
Servicing can involve cleaning, adjustment, appropriate lubrication, replacement of consumable components or a check of the sealing.
The interval is set by the environment, not by the calendar — and the signs are specific. The sash drags or will not close without pressure: usually a hinge adjustment or settlement. A draught with the window shut: a worn or deformed gasket. Haze between the panes: the seal of the glazing unit has failed, and that is cleared only by replacement. Water on the sill after rain: often blocked drainage holes rather than a faulty window. Modern hardware adjusts on three axes — height, width and closing pressure — which solves most of it; gaskets and hardware are consumables and are replaced, not repaired.