
They are not particularly expensive and are basic models working on a one fill of water/gas tank basis. This should give you a reasonable 15 min run.
I have had a water level sensor LED fitted on mine which will run on green until the water level drops. At this point it turns to red as you would expect.
The sensor 'doubles up' with a goodall valve allowing a water top up from the bottle without having to wait for the boiler to cool down.
On this basis you can run as long as you like.
This cooler weather I use the gas mix giving a better flow to the burner.