Keep it as long as your data changes. 3 months is a good amount for myself. The backups are only good for deleted documents (for me).
I always think to myself, at what point will that data still be valuable? Most of the time all the important stuff sticks around forever, and if you delete a document how long till you realised? A day? A month? Half a year?
That might help cull some of the older data.
You could always use a different host for your offsite. We use Wasabi storage which is $6-7/TB/m for Reddthat, and for object storage it's great. Or could even use B2 etc. But for personal data a good 70$/y might be out of your budget. If borgbackup can handle s3 it can handle wasabi.