There's no exact "evaporation temperature". With most liquids (maybe all?) as the temperature gradually rises, the rate of evaporation gradually rises. Even at room temperature, liquids will evaporate slowly.
The vapor pressure of the air surrounding the liquid will also be important, as if the air already saturated with vapor of the liquid, then just as much should be condensing out of the air as is evaporating into it.
Unfortunately, I'm not an expert on this stuff, and I don't know how to characterize the loss or have any figures to share. But I do know that silicone grease is used on spark plugs in cars, and car engines get up to around 200 degrees F or something, I think. And I think that'll last at least 5 years -- that's how long I've gone driving a car without doing any spark plug maintenance, and I've never had any problems with that. Of course, in that example, the grease is covered by a boot and isn't exposed to the air so much.
The boiling temperature though, is generally around 260 degrees C or 500 degrees F. That's where it isn't just gradual evaporation, but boils off quickly.
I couldn't really answer your question, but I hope this helps a bit.