Oxfordshire,
United Kingdom 
using an old dustbin filled with water to rot down marestail and couch grass how long will it take?
29 Jun, 2010
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It's partly dependent on temperature, but in a black bin, with the lid on, and in direct sunlight, not very long I would think. When I made a marestail 'tea' as a fungicide, I used it after about a month.
If you are going to put the rotted material in the general compost heap, I think I'd leave it a bit longer to be absolutely sure it can't grow again. Examine the plant tissues and if they are completely broken down then they should be safe.
29 Jun, 2010