Average Time Since Modified - Sheet Summary

I am working on a sheet and I'd like to get an average days since modified across a sheet using the system modified date column.

I came up with the following, but im getting an 'invalid data type' error.

=AVG(COLLECT(NETDAYS(Modified:Modified, TODAY()), Status:Status, "In Progress"))

I had this working utilizing a helper column that was just days since modified vs today, but that was refreshing the entire sheet every time we opened it and it checked what day it was. I guess the formula when it checks today, it 'modifies' it and then resets its value haha.

I could use help with either my formula, or with a helper column that wont trigger a modification 'event'


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