I used to use a regular cartridge razor, but switched to safety blades about a year ago. I find that I can get roughly 10 shaves out of a safety razor blade before I need to replace it, or else it starts getting dull and I start cutting myself.
The cartridge razors I used to use had a blue strip like this one to track how much 'life' the cartridge had left:
Because I work from home, I don't follow a regular shaving pattern (some weeks I shave 4 times, some weeks I don't shave at all). Thus I regularly am unsure as to how much life my blade has left. Furthermore, sometimes my wife uses my razor as well, which further makes it difficult to track (and she certainly doesn't want to write entries in a manual log).
Here is a picture of a safety razor and a corresponding blade which you load it with:
Short of writing or typing a manual entry down every time I (or my wife) use the razor, is there some trick you can suggest to keep track of how many times we've used the blade?