Check progress of alter index reorganize / rebuild
It's really hard to say how long your rebuild will take, as SQL itself doesn't really know in advance and cant give you an estimate.

You can use the following query to use the dm_exec_requests dmv to view how long your index rebuild has been going on for, and to verify that SQL doesn't really have an estimate:

SELECT r.session_id,r.command,CONVERT(NUMERIC(6,2),r.percent_complete)
AS [Percent Complete],CONVERT(VARCHAR(20),DATEADD(ms,r.estimated_completion_time,GetDate()),20) AS [ETA Completion Time],
CONVERT(NUMERIC(10,2),r.total_elapsed_time/1000.0/60.0) AS [Elapsed Min],
CONVERT(NUMERIC(10,2),r.estimated_completion_time/1000.0/60.0) AS [ETA Min],
CONVERT(NUMERIC(10,2),r.estimated_completion_time/1000.0/60.0/60.0) AS [ETA Hours],
CONVERT(VARCHAR(1000),(SELECT SUBSTRING(text,r.statement_start_offset/2,
CASE WHEN r.statement_end_offset = -1 THEN 1000 ELSE (r.statement_end_offset-r.statement_start_offset)/2 END)
FROM sys.dm_exec_sql_text(sql_handle)))
FROM sys.dm_exec_requests r WHERE command IN ('Alter Index')


However when it comes to a real estimate on time required you can read this nice blog post from sqlmunkee, which summarises this by saying "..it depends. ":


  And since we're not all on the same hardware, using the same software,
  or looking at the same data, the answer has to be… ..it depends


Frustrating, but true, sadly.