I'm thinking of setting up a flow/ebb system,but i don't no how often to flow and for how long. Can anyone help me?

Well... that all depends on how much information you provide, so far it isn't enough. Ebb and flow works off float systems if you are talking about a system with individual buckets, 5 of them actually and 2 pumps, one in the reservoir and one in the controller. If you are talking about flood and drain, which is much easier, then you can view an explanation of how to determine that, its post number 48 in my blues thread. The number of times you would water will be dependant on the medium you use, I think most use hydroton with the ebb and flow, I use coco, Al B uses foto cell and a little flock rockwool. etc.
If you give us more information we might be able too help. VV

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I was gonna do the ol' point upwards at the growfaq routine, but the section on Ebb and Flow is really lacking. Quite disappointing actually.

Anyway, you need to read up on a lot of stuff before how long your pumps are on for is a problem. Now go out to the wild world of inter-wide webs! Google until your heart is content!

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Build an ebb & flow type hydroponics system Here is a link too a DIY Ebb and Flow. All of the switches and boxes are available from them. VV

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Or you could have VV do all the leg work...

Interesting set up.... where do you store the gravity exactly?

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"I smoke two joints before I smoke two joints then I smoke two more!"