Hey there. I'm going to start with a disclaimer that I've only done a few batches myself, but I know what you're going through. I was nervous about my first batch. All of my batches started "bubbling" in the airlock within 24 hours but then have died down to little to no activity by day 3-4 which I understand to be pretty common. It doesn't mean your fermentation is complete, only that the first stage (primary) is complete. If you have a hydrometer, that is the best way to determine if fermentation is complete as your gravity will stop dropping. During primary fermentation though, I've gotten anywhere from a bubble every second or two, down to a bubble every 15-20 seconds.