How often do I need to change the filter on a siphon? Many siphons use a small piece of metal wrapped into a fine cloth. The piece of metal makes them robust, and it seems to me they can be used for a long time. The piece of cloth is usually of enough quality to remain in good condition for long. A piece of cloth can however get dirty, rough, etc.

How long do you recommend to use a siphon filter? What would be the criteria to drive for change?
I've typically used the same cloth filter on a hario syphon for several (2 - 3 probably) months of continuous use (1 or 2 syphons daily). We clean the cloth after each use using a mild detergent, hot water and a soft scrubbing brush. Rinse very well after each wash. Typically, after a week or so we soak the cloth in oxyclean or similar for a couple of hours to get it super clean and then follow this up with a normal wash and thorough rinse. The coffee will stain the cloths after the first use, so don't expect it to return to the same colour it was originally no matter how often or well you wash it.

When we first bought a syphon about five years ago we were recommended to store the filter in a glass jar of water in the fridge to prevent it drying out between uses. If we aren't planning to use the filter for a while, we toss the cloth, clean the metal part and let it dry.

You can also clean the filter (with or without the cloth) in the dishwasher but I'd still recommend giving it a thorough rinse afterwards.

The cloths are fairly cheap and in my experience easy to find online so use your judgement - if you think it's time to switch cloths, it's probably time to use a new cloth!