The physics involved in this is:
1 Watt equals 1 Joule per second
4200 (approx) Joules equals 1 KiloCalorie
1 KCal is the amount of heat required to raise 1 litre of water by 1 degree Celsius
So an answer in minutes:
(300*80*4200)/(3000*60) = 560 minutes
Assumptions:
80 degree Celsius temperature rise from 20 to 100
NO LOSS OF HEAT TO EXTERNAL
In reality the second assumption is just not going to happen and it will take a lot longer than that.