Sounds "normal" from my experience.  Seems to be temperature fluctuations that do it.  So in the cooler days/mornings it'll read orange/red then in the hot afternoon it'll go back to "full" again.

If you want to know the level of the liquid inside the bottle, pour boiling/hot water down the side of the cylinder then carefully (you just poured boiling water down it) check the the cylinder for where the temperature of the metal changes.  The liquid "gas" inside will cool the metal very quickly compared to the air(?) inside so you can get a fairly accurate level reading.

I go through between 8 and 12 bottles a year.  On demand hot water all year round and a gas flame effect heater that gets used when needed.  In summer one bottle can last up to 8 weeks.  In winter, can go through it as fast as two weeks depending on how much use the heater gets.

I also use the trick of having one bottle shut off until the first can't sustain enough gas flow for the devices.  Unfortunately that tends to be the hot water system.  And of course, always when showering on the coldest of nights.  :-/
But if I didn't do that, I'd likely forget to check the indicator until I'd run out of both.