Many people rely on the dustbag full indicator, but it’s not going to have the accuracy of, say, the gas gauge on a car. It just tells you when the bag is getting filled and “indicates” you should check it periodically, especially if you seem to be losing suction.
The thing to remember is that you should never let the dustbag fill completely. Air needs to flow through the bag for the vacuum to work properly. The fuller the bag, the less air can pass through and that reduces your suction power.
So ¾ is about as full as it should get. For a canister vacuum, if you can see dust up the sides of the bag, it’s probably time for a new one. For an upright, you can check feel how high the dust is in the bag. In either case, check before you start using the machine. Suction will compress the dust in the bag, and that may make it seem less full than it is.
If you vacuum weekly, a good rule of thumb is to change the bag once a month. You’ll need to adjust that, of course, for the size of the area being cleaned and how often you actually use the vacuum cleaner. Regardless, the one best thing you can do to keep your vacuum running at peak power is to change the bag before it gets full.