Definitely having dew sitting on the mirror for prolonged periods is bad news if there is any acidity present in the liquid pools but if the water is neutral I don’t think a long dewing is worse than a short dewing. I think the main issue with dewing is that is tends to collect up loose surface debris into rings and spots – these agglomerations of particles are then no longer loose and become stuck to the surface causing scattering and loss of contrast. The collecting action locally concentrates up any agressive debris on the surface increasing its likelihood of attack of the surface.
Fans to blow on the surface of the mirror to speed up evaporation will only work if the air being blown over the mirror is lower than the temperature of the mirror and blowing ambient air over a mirror below the dew point will just make matters worse.
Some observatory owners do have electric dehumidifiers in their observatories and these can be triggered on relative dew point or just switched on after a session to dry everything off.
Martin