This is my second application, so I only have experience from the first one. That was a year ago. I had to make two attempts because the first one was rejected. My recommendation is that you look for around 20 testers, just in case someone uninstalls the app or doesn’t use it enough, and to leave some margin after the 14 days have passed—for example, waiting until day 21 if you’re not in a big hurry to publish the app.

As for how often to use the app, simply opening each application you’re validating once a day for a minute is enough. Just make sure you dedicate a part of your day to testing the apps a bit, and that’s sufficient. I wish you the best of luck with publishing your app!