That's basically right. The -M function may make this a little easier, since it does the subtraction for you. The units are in days, so you have to figure out what fraction of a day 20 minutes is. Here's an example:

  if (-M '/tmp/file' >= 20/60/24) {
    # ...
  }
[download]

As others have mentioned, this will tell you whether the file has been unmodified for the last 20 minutes, which may or may not be exactly what you want.