It varies... Not a lot of rhyme or reason
There are definitely more offers of late that come out beyond just Friday
So worthwhile to check your Account / App a few times a week.
The majority of offers are still for 7 days (Friday to Thursday)
But, some midweek ones can be for as little as 3 or 4 days, or as much as a whole month. The average length for midweek offers tends to be 8 to 10 days (hence why they tend to expire on "odd" days)