Fit - I tried on a pair in the LAX airport I'm usually between a 7 and 7 1/2 and the 8m for woman fit better. Glad I did so I purchased the 8m and fits perfect. At 1st I thought it would be big but it's perfect.
How long can you wear? Just like any foot arch supports from the dr office or specialty stores. They take some time to get used to. I had to wear for the 1st week about 3-4 hours a day. Now they are good! Takes some time to get used to but they are definitely good arch support flip/flops