If you search the forum for "Big box store audits" you will find some other threads where more people have given their opinion on these. A lot of people dislike them, but I do at least a couple a month and enjoy it. There is a learning curve, the first audits you do will take you longer, but once you have memorized the guidelines and found a rhythm they can be done in as little as 4-5 hours if the store has few issues. That said, if a particular store has a lot of problems, you could be looking at the full 8-9 hours, as almost every individual issue has to be documented with a photo and additional information entered. You get to choose the locations you audit each month so if you do them regularly enough you will figure out which stores tend to have more problems and can simply avoid them.
As far as the tablet, you don't necessarily one to do the audits, but it helps to have the larger screen and storage space. The only requirements are that you have a device that can download the SASSIE Rover app which also has enough storage space for the required photos (anywhere from a couple dozen to a few hundred, again, depending on the number of issues), so some people are able to just use their smartphones. I think it's worth getting a tablet if you are able to, though. Amazon had Kindle Fire 7 tablets on sale for only $35 last week.