The A320 will descend around 2500-3000 fpm in open descent mode. You can control this by changing the airspeed you are descending at. Faster airspeed will increase descent rate and slower will decrease it, in open descent mode.
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In aviation, the rule of three or 3:1 rule of descent is a rule of thumb that 3 nautical miles (5.6 km) of travel should be allowed for every 1,000 feet (300 m) of descent. For example, a descent from flight level 350 would require approximately 35x3=105 nautical miles.