N95 masks are more expensive than the normal surgical mask. So, certainly, you cannot throw it after every use. You can wash it and reuse it again, but only for a limited period. How long you can use an N95 mask depends on several factors like the manufacturer specifications and how long you wear your mask in a day. The major thing to look for in an N95 mask is the seal. After several usages and washes, the seal breaks off or comes out of the mask. Once the seal is out there is no point wearing the mask as the air can pass in and out easily without filtering anything. It cannot protect the dust and virus from reaching your lungs and the entire motive of wearing a mask will be futile. Other signs based on which you should stop wearing the mask can be loose-fitting of the straps and if the mask fails to cover your face completely. In some cases, the mask does not wear out, but the seal is clogged after wearing it for a long time. You may find it difficult to breathe properly. It is also an indication that you must change your mask.