Go into your camera:
Settings --> Record and down under options you'll see
combine or cut video each and make that every 1 hour and whatever like 10 gigs.
Now go into your system settings (we'll assume you have a 200 gig hard drive and 100 gigs of free space for
Blue Iris stuff in this example):
Settings --> Clips & Archiving --> New
Limit size: 10 gigs
Uncheck the "limit clip age"
Uncheck "delete" and check "move to folder stored"
Go into "stored" and limit size: 90 gigs and select delete
What this is going to do is.... the camera will make a new clip every hour on the hour. Those hour long clips will be saved in the "new" folder and after 10 gigs of hour long clips get saved then "first in last out" will get moved to the "stored" folder and once that folder gets up to 90 gigs the oldest clip will get deleted to make room for the newest one.
What I've just explained is very far from a useful setup it is just explained that way to give your brain something to make sense of all those settings. Some people like clips every hour, some people like clips every 24 hours. Some people only have 40 gigs on their main PC and want everything after that backed up on an external HDD or a Google drive sort of cloud.
Take a swing at it and if you have questions please come back and let us know. I'll be happy to help you tweak things.