That's a great question and one I'm sure all woodworkers think about when using a plane or chisel. First you need to know what sharp feels like so you have a baseline. If you have your planes "scarry sharp" then you know how it should cut and how much pressure you need to use. As you get more experience, you will know after the first few passes if it is sharp. Cuts like butter is the ideal feel. Second, if you are getting tear-out and cutting in the right direction, then time to sharpen. Sometimes grain direction is hard to determine so try both directions.
The other factor is how heavy of a cut you are taking and the wood itself. Adjust the plane for paper thin cuts to start and adjust as needed from there.
Once you have your irons sharp, a quick swipe on the stone will keep them that way. If you wait too long, then your time on the stones go way up so in the long run, sharpening often will save you time and energy.