evaporation of engine coolant under normal condition?

No, a correct cooling system is completely sealed except for the overflow reservoir. With a hot engine, some coolant should flow into the reservoir, and then when it cools, it should be sucked back into the radiator. If my car lost one cup per month, I would be looking for the problem. There is no normal evaporation from such a closed system. The most normal way it is lost is with an over-pressure release, like from a radiator cap. Also, you could have lost coolant within the engine, but this leads to allegations about intake manifolds.

---Bob Gross---

On my 2001 Malibu, when new, a perfectly cool engine (overnight) would leave the coolant right at the Full mark, so just fine and dandy. After one and half years, this was no longer the case. I told the dealer and they checked for leaks (none, they said) and told me that "some" that evaporate, more so with the pink than with the earlier green. But the temperature needle was getting up higher than when new (never above the center mark). After two and half years, overnight, the coolant level had dipped even lower. Curiously, though, when I took it to the dealer for an oil change (two blocks away) and I showed it to him, the level was already at the Full mark... After a later overnight check, I found that even just a little bit of driving would raise it.

So I got a can of premixed PetroCanada coolant (adheres to GM standards such and such), and, while completely cool, added just enough to bring it again to the Full mark. It took maybe only about as much as one and a half cups of coffee.

Result: the temperature needle no longer goes much above the center mark, and now, when fully cooled, the coolant is AT the Full mark.

So, 2 1/2 years caused a loss like that. Is that too much? The answer is I don't know. But those are my numbers, anyway...