DECLARE @dt DATETIME, @appttime CHAR(4)
SELECT @dt = GETDATE(), @appttime = '0730'

SELECT @dt
      ,@appttime
      ,DATEADD(MINUTE, CAST(RIGHT(@appttime,2) AS INT)
               ,DATEADD(HOUR, CAST(LEFT(@appttime,2) AS INT), @dt)) 

create table #CheckIn
(
	check_in datetime,
	appt_time char(4)
)

insert #CheckIn
select '1911-03-02T14:00:00.000', '1300' union all
select '1911-03-02T14:00:00.000', '0950'

select *
from #CheckIn

;with cte as
(
	select check_in, appt_time, dateadd(n, CAST(right(appt_time, 2) as int), dateadd(hh, cast(left(appt_time, 2) as INT), dateadd(dd, datediff(dd, 0, check_in), 0))) as AppointmentTime
	from #CheckIn
)

select *, DATEDIFF(n, check_in, AppointmentTime) from cte

SELECT *, WaitTime=DATEDIFF(minute
    ,CAST(check_in AS TIME)
    ,CAST(STUFF(appt_time, 3, 0, ':') AS TIME))
FROM #CheckIn