I don't know about tractors but I do know that according to the EPA it is illegal to let a diesel truck idle over 5 minutes.
I believe that statement is partially true and also urban legend.
And:
If this statement is true, then there are a lot of highway contractors which should get fined. Dump trucks are often idled for more than 5 minutes while being loaded. Not to mention paving operations where trucks are staged and idling waiting to serve the paving machine and also while hooked to the paving machine.
It seems that people don't know how to search for information, but will discuss something with "opinions".
For example:
State Codes
Virginia Administrative Code
ARTICLE 41. EMISSION STANDARDS FOR MOBILE SOURCES (RULE 4-41)
9 VAC 5-40-5670. Motor vehicles.
C. In commercial or residential urban areas, propulsion engines of motor vehicles licensed for commercial or public service use shall not be left running for more than three minutes when the vehicle is parked, unless the propulsion engine is providing auxiliary power for other than heating or air conditioning; except that:
1. Tour buses may idle for up to 10 minutes during hot weather in order to maintain power to the air conditioning system; and
2. Diesel powered vehicles may idle for up to 10 minutes to minimize restart problems.
So in Virginia diesel can run for 10 minutes.
http://www.epa.gov/smartway/documents/420b06004.pdf
There are many different laws in different states.
There are probably many different rules for tractors, versus commercial trucks.
I used to let my '88 Maxivan idle for hours to keep an a/c "cool spot" in Mississippi.