I want to mount a cornice for curtains on a concrete ceiling. So I believe I need some kind of hammer drill. I would like to know, how powerful should it be for this kind of work?
You can drill through concrete even without a hammer drill, really it's just a function of time and energy. How many holes do you have to drill? How big and how deep? For two small holes, I wouldn't really worry about it (so what if they take 5min each?), but if I had to drill 20 large holes, I'd want something like a rotary drill.
Good bits will make quick work of the job too, and low end bits in the best drill will be slow and tedious.
For small holes (say 1/2" or less diameter, regardless of depth), I'd say use a standard hammer drill. Anything larger, rent or buy a rotary drill. For really big holes (several inches in diameter), you need a core drill.
I live in a region where most of people live in apartment buildings most of which are build of reinforced concrete.
Long time ago I used a 600-watts impact drill for drilling concrete and most of the time it was usable but really depressing - the noise is horrible, the stress on the hands is significant, the progress is slow. The real problem is if your drill bit happens to meet a stone inside concrete (like a piece of granite) - you may have a really hard time drilling through that. Later I used a 550-watts impact drill - that still was depressing, but I can't say it was more depressing that with a 600-watts drill. I've heard a lot of similar experience descriptions from people who have various other impact drills.
My latest experience is using a 550 watts pneumatic hammer (a thing with an SDS-plus chuck) - that one drills through concrete no problem. A pneumatic hammer compares to an impact drill like... well, they don't compare, a pneumatic hammer is the tool for drilling through concrete.
The bottom line is no matter how powerful an impact drill is drilling concrete is a pain. So if you only have an impact drill - bear with it. Impact drills usually start at 550 watts - that one will work - and more powerful ones will work too, but it'll be a bit of pain anyway. If you have a choice to use a pneumatic hammer - definitely go for that, no matter how "low" its power is, it will provide a much better experience. You will often hear that "that 700-watts" hammer is "a serious tool" an "this 550 watts model is "a toy", but it's mostly irrelevant for domestic use - a hammer with any power will drill through concrete no problem.