I have heard that you should let it stand for a couple of weeks to let the chemicals in the soap products to dissipate.

I would be reluctant to water vegetables with even after standing as the chemicals don't all dissipate. Then there is the question of faecal material given that you are washing all of your anatomy. Personally I'd use on flowers and round fruit trees, shrubs but not on veggies. If you do use make sure to wash them well before cooking and be extra careful around salad crops.

Ooh err, hadn't thought that far! There is an article on the RHS website detailing 'grey water' on the garden. They do suggest not using it on salad vegetables and things you can't cook and not to store for too long as it will start to pong a bit after a while.

I managed to kill off a prized pseudoacacia by watering it with bathwater back in the hot, dry summer of '76. I assumed it was the soap/detergent content that had done it.