As soon as it feels blunt.!..Yeah, 19 years a T1. I changed the needle every injection for 1 month 19 years ago. Never since!
If you re-use a needle, either by leaving it on the pen or putting it back on repeatedly, there is a risk of contamination being drawn into the insulin vial. Any bugs could then sit there for a couple of weeks happily multiplying. For a few pence, is it worth the risk?
I use one for the life time of my long acting pen and the pen and the last about 12 days . Been doing for the past 25 years the last jab feels like the first.
I've been using an Acuchek Mobile which has a built-in lancing device on cassette, which makes it much easier to flick the switch after every test so you always have a new needle for your next test.