My experience is that most of the blades i have tried feel a little better on the 2nd shave and are just starting to loose their initial sharpness after the 3rd shave, so i usually change them out after 3 shaves.
I do know from experimentation they will still give acceptable shaves after that, but have never gone beyond about 6 shaves from any one blade.
Have never experienced them losing their sharpness between shaves, BUT my belief is that soap and whisker residue can coat the edge and make them feel blunter so i do take out the blade sometimes and rinse it and CAREFULLY wipe it on back of hand ( edge away like stropping ) before putting it back in razor