12c letter

I filed my taxes with TurboTax in mid-February and received a 12 c letter asking for my documents to verify my withholdings. I faxed everything to the IRS earlier this month and called them last week. I was told they didn't see anything yet but are backlogged. How long should this take or are there any suggestions on what to do. The letter did say it could take 6-8 weeks but how do I know they even have the faxed information?