When should you throw out a pen? When the ink runs out, of course (usually in the middle of signing something very important).
When should you get a new tube of toothpaste? When you've scraped the old tube across the corner of your countertop enough times that you are 100% sure there is not a single brushing's worth left.
When is it time for a new stick of deoderant? When you've just scraped your underarm with the plastic piece beneath the last hint of product.
New socks? When your toes or heels peek through.
New razor? When the little strip looks like it's lost it's color. Hmmm...somehow the razor folks are able to get me to switch to a new razor long before it's probably necessary - just by giving me a visual cue to rely on.
Is there a way to transfer this learning to your product or service? What kind of cues can you give your customers to encourage them to repeat earlier rather than later?
The magazine industry has, of course, taken this approach too far. I just received my first renewal reminder for a magazine I just signed up for last week.