2013 – Content database more than 350GB with only one site collection

I just started a project with a customer. They have a content DB in SP2010 that is more than 350 GB and just one site collection (20000 subsites!!!). The Test-SPContentDatabase command did not complete.

But, since we are testing the migration to SP2013, I ran the Mount-SPContentDatabase command, but there is no progress indication which is usually not the case in such scenarios. Please suggest how to approach this huge task.