Can you first verify what the retention policy is in the backup plan? I would not rely on using default options for retention as they can be easily changed and would affect all backup plans. I always recommend setting them explicitly in your backup plan.

also the delay purge is not really a setting to use for retention. It's more a safeguard for delaying the actual deletion of backup data for a period of time. If you want to keep backups for 90 days you should keep them for 90 days regardless of the delay purge setting. But let's have a look at your retention settings for that hyper-v backup plan first and take it from there.

The screenshots were taken while 'editing' the backup plan. Does that mean they're in the backup plan?

those retention options look like they're in the Options dialog. I guess what you're saying is you opened up the default retention options from the wizard, is that correct? If that's the case, my recommendation is still to set your retention in the backup plan and not rely on default options. So can you set the options in the backup plan itself, and then post the new screenshot so I can make sure that they match up exactly with what you posted above?

You're right. I had 'use default retention policy' selected during the wizard and I clicked the link to configure the delay purge.

If I don't use the default retention policy, I see the options in the attached screenshot. How would I set these to get 90 days of backup retention?

If you want to keep backups for 90 days, then I would set Delete Versions Older than 90 Days. I think that would do it. Now you'll end up with 120 Days of backups before the oldest backup set can be removed - returning what's in backup storage to 90 days. I think you can leave the Number of Versions option unchecked, but I am checking with the team to make sure I've got it right. I'll reply back tomorrow with confirmation.

In the meantime, you'll need to set something if you stop using the default options - your screenshot doesn't show any retention options set when overriding. Or return to the default options until tomorrow.

I've set the retention policy to delete after 3 months. I'll check back in a couple of months to see if it's working.

Like I said, depending on the number of backups you already have in storage, it may take 120 days before the first 30 days (Full + Incrementals) are removed. But that will ensure, once you hit that mark, you always have a minimum of 90 days of backups for restore. I will still plan to reply tomorrow once I have some confirmation with the team on Hyper-V retention. Thanks.