Today my wife witnessed 3 fighter jets taking off from Teterboro Airport in NJ. She even took a video. How common is it for military jets to use civilian airports?
Fighter jets may fly into local fields for airshows, to re-fuel or for needed maintenance in route etc.
What your wife may have seen may have been related to the VIP TFR's that usually pop up in the summer due to the president (or first family) heading back to their home on weekends.
I fly out of KDYL which is fairly close (in aviation terms) and my local FBO has been warning everyone that the Secret Service has instructed them the VIP TFR is going to be common on weekends this summer.
Some airports are shared with the military. For example, Portland International in Oregon has the Portland Air National Guard Base, and Charleston International is shared with Joint Base Charleston. Airports such as these will have frequent military traffic.
Even airports with no permanent military presence may see occasional military traffic. Aircraft may do touch-and-gos there for training. It's less likely for them to stay very long, as there could be special security and ground support requirements, especially with fighter jets.
You can look at statistics for US airports on AirNav. KPDX sees military operations as 2% of its operations, while KCHS sees 20%. KTEB shows <1% of its operations are military. This is the lowest value they report but that could still be up to almost 5 per day on average.
There are many, many Air National Guard Bases at Regional and Municipal airports. The ANGB planes typically have their own ramp area and hangars for repair, and have security systems around them.
Wikipedia has a list of ANGB locations Those Not at an Air Force Base are generally sharing civil airports.