In theory, an 1800 watt heater could boil 2.5 gallons, starting at 70 F, in about a half hour.
1800 watts is 6141 btus/hours (not quit 10% of a dark star burner).
A btu is the amount of heat needed to raise 1 pound of water one degree F.
A gallon of water weighs 8.34 lbs, so in theory:
8.34 lbs/gal * 2.5 gal * 1 btu/lb - degree *(212-70)degrees = 2961 btus needed to boil 2.5 gallons
2961 btus/ 6141 btus/hr *60min/hr = 29 minutes.
That would assume all the heat goes into the water, so I would expect reality to be a bit longer.
Sorry for the math and science, but I hope this provides some perspective at least.