Transact SQL :: Notify Only If Blocking Is Happening For More Than A Minute Or 30 Seconds

I have configured an alert like below to track all blocked events in SQL Server across all databases and then kick start a sql job when a blocking happens which inserts data to a table, when there is a blocking in SQL server , i get an email  --which is working fine and i am able to track all queries.

but, HOW to get notifications ONLY if BLOCKING IS HAPPENING FOR MORE THAN 30 SECONDS OR 1 MINUTE with out using sp_configure?

---ALERT
USE [msdb]
GO
EXEC msdb.dbo.sp_update_alert @name=N'Blocking Process', 
@message_id=0, 
@severity=0, 
@enabled=1, 

[Code] .....