What is the broadcast rate of an ADS-B message?
It depends on the Message Format TYPE Code and the aircraft state. In your example, being on final approach would have no effect as the primary state criteria is being airborne (vs. surface). The rates are 'nominal' rates in that there is a programmed 'dither' to the nominal rate to avoid synchronous garble.
The nominal rate for TYPE codes 1 through 4 (ID and Type) is 5.0 sec airborne and 10.0 sec surface.
TYPE Codes 5 through 8 (Surface Position) rate is 0.5 sec in motion and 5.0 sec when stationary.
TYPE Codes 9 through 18 and 20 through 22 (Airborne Position) rate is 0.5 sec.
Type Code 19 (Airborne Velocity) rate is 0.5 sec.
Type Code 28 (Aircraft Status, e.g. emergency/priority) rate is 1.0 sec.
Type Code 31 (Aircraft Operational Status) rate is 1.7 sec.