If you want to get it deleted very quickly, I'd suggest not to rely on the timer job. I had a similar situation with a 600 GB size content database. This is the process I followed.
Everything is to be deleted permanently from the site, and not move them to Recycle Bin. This includes deletion of every components of the site, including lists, libraries, document versions, workflow history, etc.
$mySite = Get-SPSite http://sharepoint.devbox.com/sites/mysitecollection
foreach ($subsite in $mySite.AllWebs)
# this line below is responsible for deleting the subsites
Write-Host $subsite.url has been deleted successfully
If your subsites are also way too large, you may drill down inside the script a bit more and delete the large lists/library first.
Hope this helps!