As usual, it depends.
There is no warning. You will get a feel for how often it needs to be emptied based on your situation and the time of the year (or even what week it is.) I have some flowering trees. When the petals drop I need to empty the basket daily if there is any wind. Most of the spring I do it when I put the other cleaner in, maybe weekly. In the summer I may do it every couple of days if there are a bunch of flying insects getting in. If I cut the grass around the pool deck I always check it the next day.