From what you say, I understand you are bypassing the heater valve. You should be able to run this as long as you want to until you are ready to replace the valve. It won't affect your engine cooling.

When you replace the valve, you might want to do the heater hoses as well. When I "zero-miled" my 940, I did this. I lost some knuckle skin under there. Next time I think I will pull off the intake manifold so I can see what I'm doing. It's crowded under the manifold and I don't like Braille mechanicing.

How are the radiator and hoses? If you have an old plastic tanked radiator, it may be brittle and need replacement before the upper hose connection cracks off. They seem to say in the FAQ or on this board that 10 years or 100K miles is normal life span.