How do I get the watch face to stay on longer?

Unfortunately, this is not a setting you can change. The display is set to go dim after 20 seconds, and the reason is to conserve power. If your display was running constantly to display the time, the battery would be dead in no time.

I'm looking at my Apple Watch right now and the face only stays on for 10 seconds - 20 seconds would be so much more useful.

Tell Apple you would like an optional longer time: https://www.apple.com/feedback/watch.html

Just remember there will be a tradeoff in faster battery depletion.

I have timed mine repeatedly. Stays on for 7 seconds. It never stay on long enough for an app to load.

What app are you referring to on the Nano?

How to set Apple Watch to stay awake for 70 seconds

Step 1: Launch Settings from the Home screen of your Apple Watch, or ask Siri to ‘open Settings’ for you.

Step 2: Navigate to the General → Wake Screen section.

Step 3: Under the On Tap sub-section, tap on the “Wake for 70 seconds” option.