If you already printed, and then mailed your tax return (instead of e-filing), then
TurboTax actually has no way of knowing that it has been physically
mailed, or of tracking it through the postal service. As such, this explains the "ready to mail" message you're seeing in the TurboTax
software. This is perfectly normal for those filing on paper, in the
traditional manner. Many of our customers opt to file by paper, in
lieu of e-file, so yours is a fairly common situation, and question.
Unless you mailed your envelope (containing your printed and signed tax return) with the post office using Certified, Priority, or Registered mail, you won't have any way of tracking your tax return, or knowing that the IRS has received it, until it appears in their own self-reporting internal tracking system.
Once the IRS
receives your tax return, though, and opens your envelope, they will make an
entry in their internal tracking and processing system to acknowledge
its receipt. At that point in time you will know for certain that they have it in their possession.
You can monitor this process on your own, through the public viewing portal known as the "Where's My Refund" website at IRS.gov. Here is a courtesy link to that webpage:
Information is updated there as often as once a day (excepting
weekends and holidays). Similarly, if you mailed a state tax return
too, then your state department of revenue probably has a similar
website for public monitoring (most do).
Yes, you read that last sentence correctly. The IRS actually says that it can take up to 4 weeks for them to open an envelope and make an initial entry in their tracking program. If more than 21 days passes after e-filing, or more than 6 weeks after paper filing, then the IRS further instructs taxpayers to call them by telephone (at the number indicated on the Where's My Refund website).
Thus, in answer to your original question, if you know that you've already printed and physically mailed your tax return, the TurboTax software message you see is normal. (Where that is not the case, then the message is a reminder to the user that their tax return is ready to do so.)
now, the best course of action is patience. The IRS doesn't always
move as fast as taxpayers would like (especially when waiting for a
refund). We know that and we sympathize.