Terrible idea. [That's what they told Chris Columbus]
Are you in the business of selling FOOD or TIME?
This would mean that you're looking for clients who walk in, wolf down their food (to save on the bill) and then leave. [comes with the territory - these would be few, especially if target is upmarket]
I'll speak for myself here. I enjoy my food. And I hate feeling rushed while I eat. I'd hate to eat at a place where I'd be billed for every extra minute that I sat down.

And then, how would you deal with "take away food"? [can simply not offer it! Nothing wrong with saying NO TAKE-AWAY]

Also: "date traffic". [this is not the target group, so no reason to worry about them] friends who decide to meet over lunch, tea, coffee etc. If one arrives earlier, wld they want to pay per minute while waiting?

Also once ate at a place where food was billed by weight and loved the idea...Wonder whether we're disciplined enough for it to work here. [oh yes we are. it's only that no one has tried it]