BMW sets no interval for this, but my firm E36 rule is a full preventive cooling refresh around every 10 years / 100,000 miles — or sooner at any hint of trouble — and I do it as a package, not one part at a time:
- Radiator (the plastic end-tanks go brittle and crack)
- Water pump (I fit a metal-impeller pump; the early plastic impellers erode)
- Thermostat
- Expansion tank and cap
- All the hoses
These are the classic E36 overheating failure points, and replacing them together is cheap next to a warped head or a blown gasket — which is exactly what you get if you keep chasing one failure at a time.
For the refresh you'll want the radiator, a metal-impeller water pump, thermostat, expansion tank and cap, and the hoses — I've put them below.