My late father would use one twice a day for a fortnight before putting it to one side. He'd got this strange idea that the blade would somehow 'recover' after a period of time, so he'd then use it again for several days after that!
I used to use one, once a day (unless I was doing anything special in the evening), for 5 to 7 days before noticing that the blade was starting to blunt, and then throwing it away. Strangely, some people regard that as 'unhygienic', yet they'll use other razors (either with 'cartridges' or the 'cut throat' type) for far longer. I fail to see the difference (as long as the razor is washed after use).
I currently buy the cheapest disposable razors available. (Tesco: 10 for 25p). I only use a razor for a single shave, but that's because I'm battling against folliculitis (which is similar to acne, but caused by bacteria) and I'm trying to be 'ultra-clean' in my shaving habits. Otherwise I'd expect to get 3 to 5 shaves (at least) out of a cheapo razor.
Chris