Any dressing made with fresh garlic should be used quickly. If you want to keep it in the refrigerator for quite a while substitute shallots.

Also, the emulsion is going to break down. It's best to make it in smaller quantities for immediate use, as opposed to larger batches. You don't want your salad swimming in dressing anyway.

And I agree with both Susan and Amanda, but if it's good-quality extravirgin olive oil, I would never, ever put it in the refrigerator.

I agree with susan -- if it were a vinegar-based dressing, I'd say 3 or 4 days, but the flavor of lemon juice really degrades after a day. Better to make it fresh.

It's best to make the simple dressings fresh for the salad, right in the bowl, especially with lemon juice.