how long does it take for a App update to appear in the Bangle.js App Loader?

There are basically two "official" apps loaders:

1. the release version at https://banglejs.com/apps
   
2. the development version at https://espruino.github.io
   
   

The first one needs manual update from Gordon while the second one gets changes automatically by github.
So your changes should be visible on the development loader a few minutes after the merge.