Fuel trim question

Just a few questions about fuel trim. If anyone could heip it would be appreciated.

What value in the long term fuel trim triggers a check engine DTC

My long term fuel trim is usually around -7. However, recently when driving it has been jumping to -21. It will stay like that for about 20 min and then return back to -8 or -7 value. Is this something to be concerned about? It has not triggered a code and I do not see a problem with mpg.

Not knowing what year here, but the newer obd11 systems are less tolerant for long term fuel trim anamolies, if it cannot correct the values satisfactorily during a number of cycles it will set a rich or lean code. Earlier obd11 vehicles have a somewhat wider window of operation. I'd leave it alone, but it is running a little rich, which ethanol can cause, or a weak sensor, if it runs good leave it.