How to estimate REDO phase time during RESTORE
I am thinking how to estimate time of completion for this phase. But because SQL have no build-in way to do it,


If you have not enabled any trace flag you have no option but to look at DMV sys.dm_exec_requests and look for column percent_complete to gauge how much your restore operation has completed. Sample script from here 

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 ('RESTORE DATABASE','BACKUP DATABASE')



  I am restoring database now and get it hang on 100% but still going (doing redo phase).


Are you sure this is the redo phase, I think this is the UNDO phase and depending on how much VLF's are there in database log file the UNDO process can take time. Of course the whole process of restore can further be delayed if Instant file initialization is not there. You can check using This script whether database is having IFI . If not provide SQL Server service account perform volume maintenance tasks rights and this will require a SQL Server service restart

As already pointed by Gameiswar you can enable trace flags to get more information. You can also read Blog by Bob Dorr for more information about internal restore process. 

Please note these are undocumented commands so don't enable it on prod servers.. These should be used for testing purpose only

DBCC TRACEON(3004);

DBCC TRACEON(3014);

DBCC TRACEON(3604);

DBCC TRACEON(3605);

GO


Below is what I got when I restored a database with trace flags enabled

RestoreDatabase: Database logicalreads

Opening backup set

Restore: Configuration section loaded

Restore: Backup set is open

Restore: Planning begins

Halting FullText crawls on database logicalreads

Dismounting FullText catalogs

X-locking database: logicalreads

Restore: Planning complete

Restore: BeginRestore (offline) on logicalreads

Restore: PreparingContainers

Restore: Containers are ready

Restore: Restoring backup set

Restore: Transferring data to logicalreads

FileHandleCache: 0 files opened. CacheSize: 12

Restore: Data transfer complete on logicalreads

Processed 168 pages for database 'logicalreads', file 'Logicalreads' on file 1.
Processed 1 pages for database 'logicalreads', file 'Logicalreads_log' on file 1.
Restore: Backup set restored

Restore-Redo begins on database logicalreads

Rollforward complete on database logicalreads

Restore: Done with fixups

Restore: Transitioning database to ONLINE

Restore: Restarting database for ONLINE

Restore: Database is restarted

Resuming any halted fulltext crawls

Restore: Writing history records

Writing backup history records

Restore: Done with MSDB maintenance

RESTORE DATABASE successfully processed 169 pages in 0.159 seconds (8.303 MB/sec).
RestoreDatabase: Finished