Duty cycle of a radio is the percentage of time it is putting out full power. You can calculate this by averaging off time with the duty cycle of the signal you are transmitting, weighted by the amount of time for each.
The duty cycle of the signal you are transmitting depends on the modulation method, and sometimes the information being modulated.
If it's FSK or PSK, it may be 100% duty cycle.
If it's on off keying, the duty cycle will possibly vary, and you'd have to do some statistics on the source data to figure out the duty cycle.
Similarly, if it's amplitude modulation of phone, the duty cycle will vary, but there's rules of thumb for, say, talking vs. music. Obiously, a slow speaker that leaves lots of gaps will have a lower duty cycle that someone who doesn't. :)
AM modulation of other media (i.e., video) is also possible.
FM modulation is probably constant duty cycle.