Currently I can only Purge configs either before a date I stipulate or the amount of configs I'd like to keep (as per image below)

I'd like to be able to select to keep the last 3 months worth of configs regardless of how many configs I have. The issue is that with real time change detection turned on, you can end up with lots of configs for 1 device so keeping the last 40 configs may only go back to 1 month's worth rather than being able to go back 3 months.
And using the date option, it means that every week I would have to update this setting, so that it only keeps the last 3 months worth of configs.Having the option of keeping 3 months worth of configs would allow me to set and forget this purge function.