if OBJECT_ID('tempdb..#myTable') is not null drop table #myTable
create table #myTable (
   ID int primary key, 
   dt_Enter datetime,
   dt_Exit datetime);
create index ix_myTable_dts on #myTable(dt_Enter, dt_Exit);
insert #myTable  values
    (3442,'20140426 11:57:17','20140426 12:10:09')
   ,(3443,'20140426 13:08:36','20140426 14:03:14')
   ,(3444,'20140426 14:46:56','20140426 15:40:43')
   ,(3445,'20140426 16:21:17','20140426 16:53:53')
   -- ,(3446,'20140426 17:50:09','20140426 18:13:54')
   ,(3447,'20140426 18:54:05','20140426 19:09:19')
   ,(3448,'20140426 19:55:46','20140426 20:43:36')
   ,(3449,'20140426 21:29:57','20140426 22:22:35')
   ,(3450,'20140426 23:03:26','20140426 23:38:40')
   ;

declare @dt1 datetime='20140426 00:00', @dt2 datetime='20140426 19:00'

select r.dt1, r.dt2, convert(time,dateadd(SECOND, sum(tm.time_in_range),0))time_in_range
from master..spt_values v 
cross apply(select dateadd(HOUR, v.number, @dt1), dateadd(HOUR, v.number+1, @dt1))r(dt1,dt2)
cross apply(
   select 
      ID, dt_Enter, dt_Exit, 
      datediff(SECOND, dateadd(SECOND, iif(s.sb<0,0,s.sb),r.dt1), dateadd(SECOND, iif(s.se<0,0,-s.se), r.dt2))time_in_range
   from #myTable t
   cross apply(select datediff(SECOND, r.dt1, t.dt_Enter), datediff(SECOND,t.dt_Exit, r.dt2))s(sb,se)
   where t.dt_Exit>=r.dt1 and t.dt_Enter<r.dt2
   )tm
where v.type='P' and v.number<datediff(HOUR, @dt1, @dt2)
group by r.dt1, r.dt2;