Here is a reference to the governing regulation:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=cb7ed85f6c396203994cd264f44120fe&rgn=div8&view=text&node=47:1.0.1.1.14.3.240.20&idno=47
This refers to: Title 47: Telecommunication
PART 15—RADIO FREQUENCY DEVICES
Subpart C—Intentional Radiators
Radiated Emission Limits, Additional Provisions
§ 15.231 Periodic operation in the band 40.66–40.70 MHz and above 70 MHz.
Tables refer to permitted field strengths.
Two seconds per hour under certain conditions: "(3) Periodic transmissions at regular predetermined intervals are not permitted. However, polling or supervision transmissions, including data, to determine system integrity of transmitters used in security or safety applications are allowed if the total duration of transmissions does not exceed more than two seconds per hour for each transmitter. There is no limit on the number of individual transmissions, provided the total transmission time does not exceed two seconds per hour."
Exemption for weaker transmissions: "e) Intentional radiators may operate at a periodic rate exceeding that specified in paragraph (a) of this section and may be employed for any type of operation, including operation prohibited in paragraph (a) of this section, provided the intentional radiator complies with the provisions of paragraphs (b) through (d) of this section, except the field strength table in paragraph (b) of this section is replaced by the following:" [1,500 to 5,000 microvolts/meter Field strength of fundamental for 433mHz transmissions]