Hi Ifkikhar,
Firstly, unless you are in a location served by CIC itinerant services (Up North, St-Johns NL, etc.) I've never heard of the CIC handing out Oath letters at the interview, so I wouldn't recommend worrying about your wife not getting one. While the process is changing a bit in response to the new Cit Act, I'm pretty sure it's still standard practice to send an Oath invite in 2-4 months.
If you don't hear anything during that time, your wife's file may have been referred for a hearing with a Citizenship Judge. The CIC will not notify your wife that she is in queue for a citizenship test. The Call Centre can't be relied upon to interpret this accurately either. The way to figure it out is for your wife to
1. order her ATIP notes and
2. contact her MP.
The MP can enquire with CIC and find out whether she is in queue for a CJ hearing. The ATIP notes may show if she's referred to a hearing as well.
Finally, if your wife hasn't updated her Driver's License yet, I recommend she do it ASAP. It probably won't help for citizenship at this point, but it will help if she gets pulled over for any reason. If you're in Ontario she can do it online. It's easy and free.