Just change your blade when you start getting a lot of tugging. How many shaves depends on the blade, your beard, razor, shave prep, etc. I can get 10+ shaves from a Personna Med Prep. For a Feather blade, about four shaves.Thanks for that. Going by the replies on this thread I think I'll try the Gillette 7 O'Clock and the Persona blades after using up my Merkurs and giving the Feathers a go. I WON'T be using the Treet blades again. The reason for this is that they seem to 'tug' rather than cut. They just can't hack it (pun not intended) on my difficult neck hair and they ended up giving me lots of nicks. The Merkur blades perform much better in this respect....
Just wanted to note there are several different blades (confusingly) labeled as 7 O'clock. This is the Black variant:
Personna can be confusing with all the different varieties and packaging offered. The Lab Blues and Med Preps are made in Verona, VA USA and come in boxes of 100.
Treet makes a lot of different blades. The 7 Days platinum blades are actually really good: