I clean mine often, but never have washed it. It's 23" x 70", so I don't want to even try. I use a tool that pulls the thread and fabric out of the mat and then wash it with soapy water on a rag, not too wet though. It looks like brand new when I do this. I can't remember where I bought the tool, but it's awesome. It's plastic, so any piece of plastic will probably work. You just run it down the board everywhere. You'd be amazed at all the thread that thing pulls out of the mat. And washing it gets rid of marks I leave from pencils and markers that accidentally hit the mat.
I bought this big mat from an owner of a LQS. She was ordering one for her shop and I asked if she could order one for me, also. She did and at cost only. What a gal.