I normally heat it for an hour (i just press the one hour button once no timer settings so i use as and when) but could click for another hour which should make it reach 70 degrees which I think it's set at. A lot of electricity though. I haven't heated the water for four months, over the summer.
Not much cost, when spread out over that many months. In your present circumstances - I would let the water heat to maximum, then run it all off through each outlet in turn, to flush all of your pipework clear of any Legionella. I would also check to make sure there are no dead legs in the system.