It depends, you don't want to smother her, but you do want to make her feel like she's pretty or that you find her attractive. Whatever you do however, do not talk about other girls being pretty or hot in front of her because guaranteed, since she does not hear you talking about her, she will assume that you do not find her pretty or will compare herself to the girls you are talking about. In short, it is not good.
I have been in both situations - a guy called me beautiful alllll the time and it got annoying. Another guy talked about other girls being hot in front of me, which constantly made me feel inferior, insecure and like he didn't find me attractive.
In my opinion, I'd much rather prefer a guy to make a compliment on things about my personality rather than my looks. Like saying that I was funny or smart would be a much better compliment to me than saying that I look nice. It would show me that he's more genuinely interested in me as a person and isn't just thinking my appearance.
Honestly, I would love for a guy to give me compliments about my personality or other more substantial things rather than looks. I always feel a little weird, I can't say "thanks", it's not like I chose to look the way I do, you know? But I think I'm alone, or at least without too many followers, on that one.
There are next to zero girls in this world who don't appreciate being told when they look nice or how beautiful they are. If you think she is beautiful, tell her. You don't have to spend a whole evening talking about it but you should always tell people how you feel anyway. Why don't you tell them?
Tell them every once in a while, it seems more magical if you make it rare. Say it all the time and they seem to get annoyed, iritated or they just don't beleive you no more, don't say it at all and they think your wasting their time because you aren't complimenting them.
I think it's the right thing to do, but I might be alone in that opinion. Firstly, you will actually turn a girl off and push her away if she are constantly talking about how pretty she is. Not only will she feel like you are after her because of her looks, but she will feel shallow about herself. Compliment her on her style of clothes, shoes, music, etc...something she has a choice over.
And second, I hardly compliment people anyway unless I really mean it. So a pretty girl receiving a compliment from me is very unlikely. I'm somewhat cynical of people and I don't trust until they prove that I can trust them. Therefore, a girl needs to earn her compliments rather than me just throwing them out there. This may actually be hurting me but it's just whatever I guess.
there are enough guys telling a girl she looks good. Unless you have no desire to have any sexual relationship with a girl it's good to avoid telling a girl you think is good looking, if anything you want to tease her about her looks