There isn’t really any such thing as “too long” in a universal sense for everyone. You can get to where you stay plugged constantly, only removing it to clean, use the toilet, or have sex or masturbate more actively.
If you aren’t prepared for it, you might get sore eventually.
Depending on the type of lube you’re using, it may get absorbed into your body and dry up a bit, making it a little uncomfortable to remove the plug.
Gas can build up pressure until it escapes around the plug, causing discomfort or cramping as the pressure builds.
You’ll need to use the toilet eventually and may feel cramping or discomfort in a similar way when you need to do so.