Here's the math:
600w/hour = .6kwh
.6kwh x 24 hours = 14.4kwh/day
14.4kwh x 7 days (your first week running 24 hours) = 100.8kwh.
.6kwh x 18 =10.8 kwh on days running 18/6
10.8 x 23 (number of days in the month on 18/6 cycle) =248.4kwh.
100.8kwh + 248.4 kwh = 349.2kwh for just the 600w lamp...