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WITH x
AS
(
    SELECT EventID
         , MAX(EndTime) AS eID
         , StartTime
    FROM EventTimes
    GROUP BY EventID
           , StartTime
)
   , y
AS
(
    SELECT EventID
         , MIN(x.StartTime) AS stID
         , x.eID
    FROM x
    GROUP BY x.EventID
           , x.eID
)
   , z
AS
(
    SELECT y.EventID
         , y.stID
         , y.eID
         , IIF((
                   LAG(y.stID) OVER (ORDER BY y.EventID, y.stID DESC) <= y.eID
                   AND LAG(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
               OR
               (
                   LEAD(y.eID) OVER (ORDER BY y.EventID, y.stID DESC) >= y.stID
                   AND LEAD(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
             , 1
             , 0) AS grp
    FROM y
)
   , k
AS
(
    SELECT z.EventID
         , MIN(z.stID) AS stID
         , MAX(z.eID) AS eID
    FROM z
    WHERE z.grp = 1
    GROUP BY z.EventID
           , z.grp
    UNION ALL
    SELECT z.EventID
         , z.stID
         , z.eID
    FROM z
    WHERE z.grp = 0
)
SELECT EventID
     , CAST(SUM(DATEDIFF(mi, k.stID, k.eID)) AS DECIMAL(5, 1)) / 60 AS EventTime
FROM k
GROUP BY EventID;

DB Fiddle for reference

WITH x
AS
(
    SELECT EventID
         , StartTime
         , EndTime
         , IIF(
               starttime <= MAX(EndTime) OVER (PARTITION BY EventID
                                               ORDER BY StartTime
                                               ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
                                              )
               OR endtime >= MIN(starttime) OVER (PARTITION BY EventID 
                                                  ORDER BY StartTime
                                                  ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING
                                                 )
             , 1
             , 0) AS grp
    FROM dbo.EventTimes
)
   , j
AS
(
    SELECT EventID
         , MIN(starttime) AS stID
         , MAX(endtime) AS eID
    FROM x
    WHERE x.grp = 1
    GROUP BY x.EventID
           , x.grp
    UNION ALL
    SELECT x.EventID
         , x.starttime AS stID
         , x.endtime AS eID
    FROM x
    WHERE x.grp = 0
)
SELECT EventID
     , CAST(SUM(DATEDIFF(mi, j.stID, j.eID)) AS DECIMAL(5)) / 60 AS EventTime
FROM j
GROUP BY EventID;

DB Fiddle for reference

This particular solution treats time ranges that edge each other as non-overlapping, though argument could be made that they are. (e.g. 2:30 - 4:00 and 4:00 - 14:00).

WITH x
AS
(
    SELECT EventID
         , MAX(EndTime) AS eID
         , StartTime
    FROM EventTimes
    GROUP BY EventID
           , StartTime
)
   , y
AS
(
    SELECT EventID
         , MIN(x.StartTime) AS stID
         , x.eID
    FROM x
    GROUP BY x.EventID
           , x.eID
)
   , z
AS
(
    SELECT y.stID DESC) AS id
         , y.EventID
         , y.stID
         , y.eID
         , IIF((
                   LAG(y.stID) OVER (ORDER BY y.EventID, y.stID DESC) <= y.eID
                   AND LAG(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
               OR
               (
                   LEAD(y.eID) OVER (ORDER BY y.EventID, y.stID DESC) >= y.stID
                   AND LEAD(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
             , 1
             , 0) AS grp
    FROM y
)
   , k
AS
(
    SELECT z.EventID
         , MIN(z.stID) AS stID
         , MAX(z.eID) AS eID
    FROM z
    WHERE z.grp = 1
    GROUP BY z.EventID
           , z.grp
    UNION ALL
    SELECT z.EventID
         , z.stID
         , z.eID
    FROM z
    WHERE z.grp = 0
)
SELECT EventID
     , CAST(SUM(DATEDIFF(mi, k.stID, k.eID)) AS DECIMAL(5, 1)) / 60 AS EventTime
FROM k
GROUP BY EventID;

WITH x
AS
(
    SELECT EventID
         , MAX(EndTime) AS eID
         , StartTime
    FROM EventTimes
    GROUP BY EventID
           , StartTime
)
   , y
AS
(
    SELECT EventID
         , MIN(x.StartTime) AS stID
         , x.eID
    FROM x
    GROUP BY x.EventID
           , x.eID
)
   , z
AS
(
    SELECT y.EventID
         , y.stID
         , y.eID
         , IIF((
                   LAG(y.stID) OVER (ORDER BY y.EventID, y.stID DESC) <= y.eID
                   AND LAG(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
               OR
               (
                   LEAD(y.eID) OVER (ORDER BY y.EventID, y.stID DESC) >= y.stID
                   AND LEAD(y.EventID) OVER (ORDER BY y.EventID, y.stID DESC) = y.EventID
               )
             , 1
             , 0) AS grp
    FROM y
)
   , k
AS
(
    SELECT z.EventID
         , MIN(z.stID) AS stID
         , MAX(z.eID) AS eID
    FROM z
    WHERE z.grp = 1
    GROUP BY z.EventID
           , z.grp
    UNION ALL
    SELECT z.EventID
         , z.stID
         , z.eID
    FROM z
    WHERE z.grp = 0
)
SELECT EventID
     , CAST(SUM(DATEDIFF(mi, k.stID, k.eID)) AS DECIMAL(5, 1)) / 60 AS EventTime
FROM k
GROUP BY EventID;