Some say you should change every prick, some change once a day or week, and some whenever daylight savings changes occur

It was suggested at clinic that we change the lancet whenever we opened a new lots of strips, so thats at least every 50 tests.
Lancets aren't subsidised, but they aren't expensive either. I think you can buy a packet of 10 from the pharmacy?
If you have trouble getting blood then I would change the lancet. We use the Multiclix and I just roll it on whenever I feel the old one has been used enough.