How do you lengthen the amount of time that Time Machine keeps hourly backups? Time Machine keeps hourly backups for 24 hours by default before purging for daily backups and I'd like to extend this further (to a week of hourly backups). Is there a way to change the backup retention policy config to allow for this? There doesn't seem to be an option in the UI.

Failing this, I'll have to install something else and I'd rather stick with Time Machine because of a fairly large initial backup (millions of files, TB's of data, already under backup with Time Machine)
The answer is this isn't easily done. Apple doesn't expose the timing which means it's likely to change if you apply any updates to the system or the change might even break backups so they don't run.

I've left a comment asking about the use case. I've employed scripts to get around this in several cases for customers, but I'd need to know more about your actual use case before providing an alternate solution that would work even with the default Time Machine scheduling.