I have two questions. First, what is considered the average number of shaves from an SE blade? Second, are there any methods to increase how long a blade lasts?
Blade length is 100% dependent on the blade and your face. certain blades lose their edge MUCH faster than others and your face may tolerate varying levels of dullness from different blades. You'll just need to test and find out.
Secondly: Making sure your blade is dry after you shave is the best you can do to make sure it lasts. Pat/shake dry (do not rub) or dip in rubbing alcohol and you should be alright. Don't leave the blade in the razor as that will reduce air flow to the blade and it won't dry properly.