Find time between one fixed row and one variable row

Hi Anonymous,

It looks like there existing more than one records where Action is set to "Set Priority to regular" per Ticket ID. If so, please try below suggestion. 

First, add a calculated column in Table1.

Index =
RANKX (
    FILTER ( 'Table1', Table1[TicketID] = EARLIER ( Table1[TicketID] ) ),
    Table1[Action date/time],
    ,
    ASC,
    DENSE
)

Then, add measure [Average] into a card visual.

Timediff =
VAR _CurrentActiontime =
    SELECTEDVALUE ( Table1[Action date/time] )
VAR _NextActiontime =
    CALCULATE (
        SELECTEDVALUE ( Table1[Action date/time] ),
        FILTER (
            ALLEXCEPT ( Table1, Table1[TicketID] ),
            Table1[Index]
                = MAX ( Table1[Index] ) + 1
        )
    )
VAR diff =
    IF (
        SELECTEDVALUE ( Table1[Action] ) = "Set priority to regular",
        DATEDIFF ( _CurrentActiontime, _NextActiontime, SECOND ),
        BLANK ()
    )
RETURN
    diff

Average =
AVERAGEX ( ALLSELECTED ( Table1 ), [Timediff] )

Best regards,

Yuliana Gu