Controlling program Autodelete

I have an older Cisco TiVo box connects via cable rather than a more modern WiFi One.

Its got about 15% capacity left but many programs end up gettijng the dredded "Will be deleted in24hrs" message

But i do not know what parameters it uses to decide that.

Is it how old somehtinsg is? 

or does the box "think" there is X number of hours capacity left and X plus 10 hours of recordings scheduled i'd better delete somehtings.

But neither make sense as some programs are not very old a week or so and not that many recordings are sceduled within that 24 hour period.

So i end up having to set more or less everything i'm not going to watch almost immediatly "keep until i delete" which is a bit of a pain having to constantly monitor all recording.

Anyone know how it decides what to delete.

Cheers  Keith