1 - The Move-SPSite cmdlet moves the data in the specified site collection from its current content database to the content database specified by the DestinationDatabase parameter. A no-access lock is applied to the site collection to prevent users from altering data within the site collection while the move is taking place. Once the move is complete, the site collection is returned to its original lock state. The destination content database specified must already exist, must be attached to the same SQL Server as the site collection's current content database, and must be attached to the site collection's current Web application.
2 - I don't think we can find the exact time. But there won't be any down time. Users can still access the site, but won't be able to modify till the move completes.
PowerShell commands required
Find disk usage (Optional)
$used = (Get-SPSiteAdministration -Identity <http://ServerName/Sites/SiteName>).DiskUsed
Trim audit data (Optional)
(Get-SPSite -Identity <http://ServerName/Sites/SiteName>).Audit.TrimAuditLog(deleteEndDate)
Moving site collections between content databases
Move-SPSite <http://ServerName/Sites/SiteName> -DestinationDatabase <DestinationContentDb>