SSMS Activity Monitor Expensive Query start time
There are no such options!

However, you can create your own result set as shown in the following snippet. 

Please, notice a couple of things. 

First, you can change a period of time you are interested in by changing the first line of the snippet. 

Second, the WHERE clause is applied on the last_execution_time column. 

And, finally you can change the order if you are interested in expensive queries based on CPU or logical writes. 

DECLARE @filterDateTime DATETIME = '2019-10-16 11:09:00.000'

SELECT TOP 10 SUBSTRING(qt.TEXT, (qs.statement_start_offset / 2) + 1, ((CASE qs.statement_end_offset
                                                                            WHEN-1
                                                                            THEN DATALENGTH(qt.TEXT)
                                                                            ELSE qs.statement_end_offset
                                                                        END - qs.statement_start_offset) / 2) + 1), 
              qs.execution_count, 
              qs.total_logical_reads, 
              qs.last_logical_reads, 
              qs.total_logical_writes, 
              qs.last_logical_writes, 
              qs.total_worker_time, 
              qs.last_worker_time, 
              qs.total_elapsed_time / 1000000 total_elapsed_time_in_S, 
              qs.last_elapsed_time / 1000000 last_elapsed_time_in_S, 
              qs.last_execution_time, 
              qp.query_plan
FROM sys.dm_exec_query_stats qs
     CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
     CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
WHERE last_execution_time &gt;= @filterDateTime
ORDER BY qs.total_logical_reads DESC; -- logical reads
-- ORDER BY qs.total_logical_writes DESC -- logical writes
-- ORDER BY qs.total_worker_time DESC -- CPU time