As of
last summer, stations airing programming from the "big four" networks (ABC, CBS, Fox, NBC) in the top 25 markets are required to have SAP channels and broadcast at least 50 hours of descriptive video services on them each quarter. Due to this new stipulation, the majority of primetime scripted programming on ABC, Fox, and NBC is now being audio described. CBS is still lagging a bit behind in providing audio described programming, with only a handful of scripted shows on the network providing the service at the present time. Wikipedia has a list
here of which network series are audio described.
Now, outside of the top 25 markets, SAP services, as evidenced by this thread, can be very hit and miss on the "big four" networks. Starting in the middle of 2015, however, the requirement to offer DVS ramps up to the top 60 markets and will likely increase to include more markets in years after that.
PBS is not included in the requirement, but that's not much of a problem since PBS stations, especially those in the largest markets, have a long history of voluntarily providing SAP services such as DVS or an alternate program language, most often Spanish. Speaking of which, now that the major broadcast networks in the top 25 markets have SAP channels, there has been more use of them during sporting events to broadcast commentary in Spanish. News and talk shows are increasingly becoming the only network programming that universally does
not make use of SAP services.