Posted: 29/04/2013 at 13:22
I have read your earlier posts and understand that you don't want to use glyphosate. I would think that a totally obscure material such as black polythene would kill your grass in around a month or two, though some aggressive grasses such as couch could take a lot longer. Weed suppressing membrane is different. I have just checked my own (in the shed!) and it does let some light through ( as well as moisture of course), which means the grass could take a lot longer to die, but I have no idea how long that might be.
If you have perennial weeds such as dandelions, they will take a lot longer to die. They survive underground every winter!