This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
Import Threshold Time setting in Editor Preferences does not accept values with a leading decimal point (.25 vs 0.25).
UEditorLoadingSavingSettings::AutoReimportThreshold in EditorLoadingSavingSettings.h
Tested in binary distribution of 4.12, and a recent build of dev-platform.
Edit
Editor Preferences
Loading & Saving
Auto Reimport
Import Threshold Time
Set the "Import Threshold Time" to a new value with a leading-decimal ex '.25' rather than '0.25'
Hit Enter
Expected: value set to new value ex 0.25
Actual: input ignored, value reset to pre-existing value
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34511 in the post.