Mine is 1Kw on low....1.5 on med and 2KW on high, but the thermostat turns on/off once it's heated, so this is dependant on the size of the heater, and the outside temp.
Mine usually takes about 30mins to get up to temp (by this, I mean the oil inside has heated sufficiently), then turns on/off every 10mins or so.
So for 2KW, at 20 hours a month at 12p per KwH - it will cost you £4.80 for the month roughly.
As i said above, ours was on for about 70-75 hours (at 2Kw) in Jan last year - It increased our bill by about £20 (for that month alaone) - with another £30 for Feb and Mar.