There are comics where the authors can count on me to comment on every single update (unless I'm really that busy and even then I'll try my best). They're often the ones where I feel a certain attachment and where at least a little part of me would die if the comic stopped updating, for example if the author is friends with me, someone I look up to or if they really, really drew me in with their characters and story. Many times those criteria overlap.
And then there are comics where I comment every now and then, almost never or haven't even left a single comment yet. That doesn't mean I don't like them (I wouldn't subscribe to something I don't enjoy!), it just means that maybe I haven't gotten super attached to it yet or that I'm still trying to figure out if the author updates regularly or if it's going to be another one of those webcomics that stop updating after two pages, or... Basically, attachment and trust are important for me so that I can comfortably comment on people's work regularly.
Overall, though, I try to leave as many comments as I can with whatever free time I have left because I know just how much getting a comment means to me, too! A while back I always considered it really hard to find something to say, but practice makes perfect. Even if it's "only" pointing out a particular expression I like or some line I thought was super witty--there's always something to say, and even if it's been said before, I don't think authors ever get tired of compliments. 