ALTER TABLE command with no default value taking forever?
Why would it be taking so long?


You can look at the Session Wait Stats to see. eg

declare @session_id int = 121 

select top 10 *
from sys.dm_exec_session_wait_stats
where session_id = @session_id
order by wait_time_ms desc

select command, wait_type, wait_time, last_wait_type, cpu_time, writes, total_elapsed_time
from sys.dm_exec_requests 
where session_id = @session_id 


where 121 is the session id of the session running the query.


  Why would it be taking so long if I'm not updating the existing records?


It probably wouldn't.  Changing the nullability is a change to the data type, not a constraint.  And it appears that it requires a rewrite of your table.