How much lumens to keep plants from flowering?

I've been doing some research but haven't found the answer so hopefully this is a new question that someone will be able to help answer. What's the minimum lumen required (per sq ft?) to keep a plant from going into flowering? Keep in mind I asked nothing about growing. My plan is that once August approach and plants start to flower I'm going to give my clones in the greenhouse an extra 3 hours of light per day with CFLs to keep them vegging til August. If anyone can help answer this question I would appreciate it. Thanks.

I'm not sure I understand the question. It isn't the amount of lumens a plant is exposed to which induces flowering, it's the amount of time during a 24 hour period the plant is exposed to light.

In order to keep your plants in a vegetative state, keep lights on them for 18 hours (or more) per day. When you're ready to flower them, drop the amount of time under lights to 12 hours per day.

...having re-read your post a few times, are you asking what is the minimum amount of light required to prevent a plant from flowering, assuming it's in a dark greenhouse? A small amount. A 20W bulb an inch or two over each plant is sufficient.

Thanks SunnyJim. But wouldn't bulbs with more watt (lumens) be required as the plants gets larger? And just so you know, all my young clones are being tie down using the low stress training method.

I would hazard a guess and say you would need 1000 lumens per plant minimum. And yes, as the plant gets larger, it's always a good idea to add as many watts/lumens (of the right spectrum) as you can afford.