To roughly(best case scenario) calculate you'd need to know how much water you are trying to increase the temp of.
You can do a real back of the envelope calculation, not taking into account heat loss, which would depend on your level of insulation. There is also no account for the efficiency in your heater.
If we know 1BTU is required to raise 1lb of water 1 degree F, and we know 1 watt = 3.4 BTU/hr
If say you had 5 gallons of water, you wanted to raise from 60-170F, that would be 5 gallons * 8.3lb/gallon raising 110F so thats 110F*41.5lb = 4565 BTUs.
If your 2000watt element is 2000watts * 3.4 BTUs/watt, that would be 6800 BTU/hr.
So knowing you need 4565BTUs and you can provide 6800BTU/hr, dividing the two, you'd get 4565 BTU/6800 BTU/hr = .67hrs or about 40min.
Again this is not taking into account heat lost, but it should give you a ball park.