Assuming you have a US passport, you'll be able to stop over for up to 48 hours in each direction without a visa. I think 2 hours is safe, especially on the west bound... for the other direction, you should check on the on time stats for your HK-SH flight; some of them are pretty poor. If you're determined to avoid the (infinitely more convenient) nonstop on this route, you might as well do a true stopover or two, and maybe catch Beijing instead in one direction (24 hour max there without visa).