The lifetime of a common house fly is a very common misconception among the general public. I remember, as a kid, being told in school that house flies will live a day, maybe two at most, before dying. Alas, this is another misleading statement that isn't true. The common house fly doesn't live very long, but lives longer than a day.
I checked out a few sources and decided to post the link from a insect/pest expert, Orkin. According to Orkin, the common house fly can live up to 30 days and reaches the maximum number of days alive when temperatures are warmer.
How Long Do Flies Live?
This explains why that pesky house fly seems to be flying around a room for days. Because it is!!! It's not a new fly each day. It's the same fly that you just haven't gotten rid of yet!