Hello Balaji,
Yes I have done ALE migrations having more than 2 lakh objects. It took me a few days 
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ALE is very simple to setup as well. You could use the standard DOCMAS.
Do you have a content server for your originals ? Please see this SAP note, 1027056 - ALE-ContentServer Scenarios related to DMS migrations.
I would suggest you migrate them in batches starting with a batch of size X and then gradually increasing the batch size. Example you could first migrate 10000 documents then 20000 and so on.
It is suggested you do this on weekends or non working days so as to avoid any unnecessary load and slow down the SAP server. As a best practice you could ask your BASIS consultant to monitor the system load when you are doing migration and inform you if there is any problem.
Hope this clarifies.
How to close a discussion and why
Best Regards,
Ameya B.