;with cte_max as
(
select dateadd(mi, -15, max(StatusEndDateTime)) as EndTime, min(StatusSetDateTime) as StartTime
from AgentActivityLog
), times as
(
select StartTime as Time from cte_max
union all
select dateadd(mi, 15, c.Time)
from times as c
cross join cte_max as cm
where c.Time <= cm.EndTime
)
select
t.Time, A.UserID, A.Status,
case
when t.Time = A.StatusEndDateTime then 0
else A.StatusDuration / (count(*) over (partition by A.StatusSetDateTime, A.UserID, A.Status) - 1)
end as Duration
from AgentActivityLog as A
left outer join times as t on t.Time >= A.StatusSetDateTime and t.Time <= A.StatusEndDateTime
SQL-Fiddle-Demo
