Always and often. Especially when I can tell someone their child / spouse did something outstanding.
The minyan maariv I daven at had a group of 12 year olds coming for a few months, and I told each of the fathers (who, for whatever reason, did not daven at that minyan) how nice the kids interact with each other before and after davening.
In another instance, I told someone what a warm greeting his wife gave my wife (I said she greeted her like a sister), and how lucky he is to have a “bnei bayis” with such a simchas ha’chayim. (it was a very chassidish person, so I had to choose my words carefully).
3rd instance (and then I’ll stop) – I told someone, “now I know why you are zoiche to such erliche children (his kids are dolls). Why? Because I saw you duck outside (after olenu) to take / make a call on your cell phone.
In short, everyone I can praise, I do. You’d be surprised at how easy it becomes once you start living that mindset.