I don't think you have anything like this length of time for the average consumer. Pop into a book shop to see what I mean. Assuming you're unknown you have to first pass the cover challenge. This is a bit like mechanical people telling you the colour of the car doesn't influence their decision to purchase, in that it's a lie!
Assuming, your cover has enticed them to actually pick the book up, you've got either the back cover/inlay or the first paragraph of text inside (those reading the cover/inlay blurb will usually progress to the innards) to hook them.
If that draws them in, they either decide to buy there, any read on for 2 or 3 pages total (but the vast majority decide way before this point).
So, in percentage terms.... less than 1% is your first target.
Once they've actually bought it they will commit much more time to give you a chance, and depending on several socio-economic factors you can have as much as 3 chapters to grab them.
Assuming you've cleared all these obstacles, and someone ditches the book after all that (that's called effective marketing) it's a badly written book.