every 6 months if used very heavily, every 12 months if not used as much. membrane should be replaced every 3 to 4 years. a good place to get them at is bulkreefsupply.com
The TDS meter? Or the membranes? The membranes, I assume. It depends on your preference. Some people (like me) think that anything below 20 is doable. Other people wouldn't let their TDS get above 5.