I agree with the others; the gauge is useless to us. Some folks have had luck with theirs but we haven't and it's not worth the time, effort or expense to try to keep them working. Went for decades without them and don't see it as an issue.
You can/will learn about how long you can go before dumping if you follow pretty much the same pattern with the same people. You can look as suggested, listen (it will burp in the toilet when getting full - at least ours does/has) or dump on a schedule. When connected to full hookups we go 3 days and dump on the morning of the 4th. The primary driver for that is showering; we don't take "navy" showers so it fills pretty quickly. Due to that I usually dump quite a bit of water into the black tank over that 3 days because it won't fill unless I do and I like the extra water in it to help it "flow".
I think our tanks are about 30 gal. if I recall??