That question gets asked a lot by newcomers to the Shavette shaving technique. I am sorry to say, but it is hard to give a good answer. I could give you a number of shaves after which you should change the blade to a brand new razor blade, but that would not hold true. For some people who have thicker beard they should change more often than people with light beard.
For me a blade is good for about 5 to 7 shaves. This goes for both Shavette blades and safety razors. I change the blade whenever I feel that it is getting unsharp. If you clean the blade really good after every shave and prepare your beard with good shaving oil and hot water before shaving it could extend the life of the razor blade with a couple of shaves.
Some people like to change their blades almost after every shave, but some other report that they could use a blade for more than twenty shaves so I guess this is really individual.