The higher mAh, the longer the battery life. Typically, the current drain during search is around 30 mA(milliamps). Once a signal is received, the audio circuitry draws much more additional current.
Typical 9 volt alkaline batteries are around 500 mAh. And if your nimh batteries are rated at 175 mAh, You will get only .35 (from dividing 175/500) the rated alkaline battery life which is listed in your CZ manual.
mAh(milliamp hours) rating isn't the maximum current the battery can supply to the application. It is more a rating of usefull life of the battery. For instance, if your device draws 500 mA of current @ 9 volts, and you use a 9v battery rated at 500mAh, the device should run for 1 hour before the battery is no longer useful. If your device only draws 100 mA, the battery should last 5 hours.
Another thing to realize is, since the individual cell voltage of nimh's is 1.2v compared to 1.5 volts seen in alkaline batteries, many 9 volt nimh batteries only supply 7.2 volts. This gives a total series voltage of 14.4 volts, where two alkaline batteries will supply 18 volts to the CZ. This is fine since the CZ's regulators supply 10 and 5 volts to the CZ circuit. But instead of 18 volts dropping to 10 during useful battery life cycle, the nimh's will start out at only 14.4 volts. This *may* further hasten the discharge cycle time of the nimh batteries.
Some newer 9v nimh batteries use 1 or 2 additional cells in their cell banks, and can supply 8.4 and 9.6 volts. These should give a longer discharge cycle in your CZ6, compared to batteries rated at only 7.2 volts @ the same mAh rating.