You can flower any one of them at any time. It all depends on what you are trying to do, and what kind of lights you are using.

Yours look like they are about 4 to 6 inches high, and if they are from seeds, you don't know which are male and which are female.

So, if you are gonna use one or two as a mother, you obviously cannot at this stage since they are not sexed. What is usually done, is letting your plants grow a bit more and then take a cutting, let it root, and as soon as its well established, mark that plant so you know where it came from and flower it. It will show its sex within 10 to 14 days. If its male you can throw both that plant and the one you cut it from in the garbage. If its female, then you can seriously start taking cuttings from that main plant and root some more of those.
When you put a female plant into flowering remember that the plant will end up about 3 times the size it was when you started it in flowering. So the question is, how much head-room do you have for it? If you have lots of head room, but only a small light, you're best off flowering them when they are smaller since the light does not have that much penetrating power such as a 1000W HPS light.
Hope that helps

SN