That depends on temperature, particle size, and chemistry. Most of the filter bags we sell are made from polypropylene felt (PO), so start there. For higher temperatures most people go with polyester felt (PE). For more demanding applications we usually end up recommending absolute rated nylon mesh (NMO).

Usually because they are working harder. Bag life is limited by the amount of dirt captured, not by hours in service or gallons processed. bag filtering clean water may last a year. A bag filtering mud may only last a few minutes. A product can appear clean to the naked eye and still be full of particles that are too small to see.

Watch the inlet pressure. The pressure drop across a bag rises as the active surface area is used up. When the pressure start to rise faster the bag is almost used up and should be changed. Every application is different, but a good rule of thumb is to change bags when the you see an 8-12 PSI pressure drop (the difference in pressure between the filter's inlet and outlet).

Filter bags are designed for one time use. Attempting to clean a bag will stretch the material. Some holes will get bigger and allow dirt to get through. Other holes will get smaller, plug up faster, and start removing things from the product that are supposed to stay in it.

In NMO100P2S the 'P' stands for plain. Until the advent of modern glazed felt it was common for filter bags to have an outer cover. You can still special order PENF10M2S bags which have a muslin cloth cover.