From Five Star DEF's Library of Useful Information on Clean Diesel Technology:
Excessive idling is defined by each state in regulatory protocols that are intended to reduce diesel emissions as required by the 1970 Clean Air Act as amended by the EPA from 2003-2013. You can find each state's legal definition of excessive idling here, and indeed "it depends"
30 of 50 states and DC have implemented laws that restrict idling of diesel vehicles; expect the remaining 20 to do the same soon. As usual, state lawmakers have made the "simple" into something complex, so many of the individual laws are very complex, and often different. The statutes are not uniformly enforced, but large fines have been levied in highly publicized cases. Expect enforcement to be significant during air quality crisis events that occasionally occur in urban areas.
A summary of penalties:
fines vary from a warning to over $25,000
fine is unlimited.
A typical penalty is $50 to $100. Expect enforcement behavior to be motivated by revenue concerns in some areas.
Drivers and owners are equally liable in some states, and both can be fined.
The intent of the EPA legislation is to limit in-city idling by Class 8 Trucks (18 wheelers) and urban transportation buses that can have a serious effect on air quality in urban areas.
There are many exemptions to the idling limits, mostly oriented to situations beyond the driver's control such as weather, traffic, maintenance, etc.
In some states, clean diesels such as those made after 2013 are exempt from idling restriction because they are so clean. In some areas during an air quality crisis with very high levels of particulate matter, clean diesels emit lower levels of particulate matter than they take in.
Summary: The "common sense" definition distilled from a review of all this legalese is 5 minutes or less
per hour in ordinary circumstances. Then again, I'm not a lawyer, so it is up to you.
I've attached the ATRI's handy reference card on this subject for those who are interested in the fine details of this law, and want to be fully informed in each state they RV in, although I think everyone on this forum has better things to do with their time
I shut down our F350 after 60 seconds or so after a long highway run at high load to allow cool-down. I warm it up for 60 seconds or so when it is first started. Otherwise, we don't idle a lot. Idling wastes fuel, and tends to load up the DPF, resulting in unnecessary active regeneration that burns extra fuel.
As always, please consider Five Star DEF an IRV2 resource on Clean Diesels, especially Diesel Exhaust Fluid.