I suppose it would depend on the blade and your beard but...I use derby extra and replace every 2 to 3 shaves, replacing when it isn't gliding smoothly and comfortably.
I use the Feather super pros & get roughly around 8 shaves per blade on average but I change blades prematurely. Just go by feel, when it starts to pull a bit more than usual change blades..the more you use it the more you will get the feel of what we mean
I think it is compariable to a DE or SE razor. I have not tried to use my DE shavette on consecutive days enough times to establish a good baseline.
With Feather AC I can get 2 weeks and usually 3 weeks or more. The blade loses some of its initial keenness after a few shaves but I find that it continues to give many good shaves afterwards. I change the blade when I realize that I am starting to use too much pressure relative to a new blade.
The sign for me that I need to either change the blade on a shavette or hone my straight is irritation or difficulty getting the hair on the edge of my chin cleanly. They are the toughest hairs for me and if a blade is not optimum I will need to make multiple passes and pressure in this area. If a blade is nice a sharp I just glide over and hair is gone!
Best rule for me though. When in doubt, change it out!