Average time duration between events

I think you may want to review your expected result by area.

Avg time between events =
VAR a =
    SUMMARIZE (
        'Table',
        [Area Code],
        'Table'[Store_ID],
        [Activity TimesStamp],
        "Prev",
            VAR a = [Area Code]
            VAR s = [Store_ID]
            VAR t = [Activity TimesStamp]
            RETURN
                CALCULATE (
                    MAX ( 'Table'[Activity TimesStamp] ),
                    'Table'[Area Code] = a,
                    'Table'[Store_ID] = s,
                    'Table'[Activity TimesStamp] < t
                )
    )
VAR b =
    ADDCOLUMNS (
        a,
        "diff",
            IF ( ISBLANK ( [Prev] ), 0, ( [Activity TimesStamp] - [prev] ) * 86400 )
    )
VAR c =
    FILTER ( b, [diff] > 0 )
RETURN
    AVERAGEX ( c, [diff] )