How long does an electric stove take from one heat level to the next one (provided an average pot is full of water)?

They vary considerably. Induction hobs are fastest halogen hobs are also fairly fast and will vary depending on the wattage of the hob. Obviously a 2kw hob is going to heat a pan of water faster than a 500w hob. Exposed element rings are faster than enclosed ones but have hotspots.