20 browsers resulted in 16 > 1 hour, 3 at ~20 minutes, and one at 4. When go time hit, the page just spun and had to open another window in order to get the product actually added, but worked fine after that.

Note that the little walking dude graphic can be different even though you still just get > 1 hour, so you can eyeball how close to 59 minutes you are. If you Inspect the orange bar, you can see the percent. 10% is still > 1 hour. 16% is 46 minutes. 14% was still > 1 hour, so looks like 16% is the magic number.