When I use: DATEDIFF(HH,Start.WorkQueueStartWorkDate,Finish.WorkQueueEndWorkDate) RunTimeInHours
I get 20 Hrs

When I use convert(varchar,(Finish.WorkQueueEndWorkDate - Start.WorkQueueStartWorkDate),108) RunTime 
I get 04:29:56

convert(varchar(5),DateDiff(s, Start.WorkQueueStartWorkDate,Finish.WorkQueueEndWorkDate)/3600)+':'+convert(varchar(5),DateDiff(s, Start.WorkQueueStartWorkDate,Finish.WorkQueueEndWorkDate)%3600/60)+':'+convert(varchar(5),(DateDiff(s, Start.WorkQueueStartWorkDate,Finish.WorkQueueEndWorkDate)%60)) as [hh:mm:ss]

SELECT 
    CAST(DATEDIFF(SECOND, Start.WorkQueueStartWorkDate, Finish.WorkQueueEndWorkDate) / 3600 AS varchar(3)) +
    RIGHT(CONVERT(char(8), Finish.WorkQueueEndWorkDate - Start.WorkQueueStartWorkDate, 8), 6)
from (
    select CAST('20121216 20:57:01.293' AS datetime) AS WorkQueueStartWorkDate
) AS start
cross join (
    select CAST('20121218 01:26:57.573' AS datetime) AS WorkQueueEndWorkDate
) as finish