So here is an example of what it looks like.
We can see KeepMessageForDays zero.
If the value is zero, it means forever.
It's the default value.
So up here we can see zero equals forever,
365 is one year, 30 days equals 30 for iOS 14 plus.
For iOS 13 and below, if you search for KeepForDays
and you have zero hits, the null value meant forever,
365 was one year, and 30 was 30 days.
So make sure you check that.
It's very simple to examine it.
What I suggest is that you just search for the word keep when
you're searching that plist, and then go and look at how long
its KeepMessage.