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 think the secret is being very slovenly. Everything goes in the dishwasher, except the good china and cut glass which is very rarely used. Anything that does get washed using washing up liquid is done asian style by DH which I expect uses less. (Couple of drops of washing up liquid on a wet sponge, scrub everything then rinse in running water.)
I tend not to wash up at all because Iamlazy it gives me contact dermatitis.
My washing up liquid lasts forever. In fact, I think the most I ever got through was when I decided it might be fun to make my hair stand up into a giant spike with a handful of Dax wax. I regretted that, I can tell you