We do it for every drink; for those who are particularly good and/or attentive, it may increase over the cruise. Anything around 5-15% on the total bill (after the auto gratuity) depending on whether we went to the bar ourselves, got the pre-made drink of the day brought around (not too common for us), got a more complicated drink like some of the speciality coffees, asked them to hunt down something specific, were in a nice lounge, or they went out of their way to do something for us. I'm far too lazy to make cocktails at home even if I have the ingredients, let alone a speciality coffee - I can barely make espresso - so I appreciate someone doing it for me and that gets rewarded a little more. If it's more complicated than a glass and a bottle, I pretty much just don't do it. So I like to indulge on vacation.
It's not a lot per drink (sometimes as little as $0.75 or less or as high as $2/drink for something expensive/complicated) but we often have a speciality coffee early in the day, a drink in the afternoon and a pre-dinner cocktail plus at least one glass of wine at dinner times two people. For us, it's only about $100-200 per cruise depending on the length of the cruise. That's also why it's useful to try to get the same servers. If you use several different servers, each one might only get a dollar or two from you but if you frequent the same person, they might get $20-30. Not enough to send their kids to college but enough that it's remembered especially if not everyone does it.