Describe the bug
Right now, this string breaks Pontoon in Welsh, as lifting the selectors to the message's top level results in 6^4 = 1296 variants. It's also unnecessarily complex, given that its input values are and have always been hard-coded:

To Reproduce
Steps to reproduce the behavior:

1. Go to https://pontoon.mozilla.org/ru/common-voice/web/locales/en/messages.ftl/?string=213590
2. Observe that an excessive number of edit boxes are presented.
3. Go to https://pontoon.mozilla.org/cy/common-voice/web/locales/en/messages.ftl/?string=213590
4. Observe that the browser freezes, and eventually asks the user to stop the script.

Expected behavior
The message should not presume that the fixed inputs are actually variable. Here's a valid alternative that's much easier to translate:

activity-needed-calculation-plural =
  10,000 hours is achievable in just over 6 months if 1,000 people record 45 clips a day.

Screenshots

Additional context
mozilla/pontoon#2703

It should not count as a feature that this particular string is one we tend to use to stress-test Fluent, as it's the most complex one in all of Mozilla's localizations.