youll have to have it running all the time, otherwise your temps will look like a saw wave:
as you can not really see due to my lack of skill, if you let it run to cool the water all the way down, itll stop running. the cpu will warm the water until it hits the critical point, then the cooler will kick back in, and itll drop. expansion contraction expansion contraction just doesnt ever seem like a good idea during watercooling, but it might just be me. what this also means is you wont be able to hold constant temps without it running all the time, which as you said may decrease the MTBF.