The last time I bought washing-up liquid it was on a 3 for 2 offer. I can't remember exactly when I bought it but it was before the closing date of the competition on the back label, which was February 2007. I have an inch left in the second bottle and one full bottle under the sink. By my rough calculation I'm using about a bottle every 18 months, although I must have used quite a bit during the month that it took to replace my broken dishwasher.
I'm unaccountably proud of this.