There are days when there is 14 or 15 hours of light. There are days when there is 10-11 hours of light. Adjusting the amount of light a plant gets within a one day period may not upset your plant too much.

Or it might f--- it up big time. oldmac is right. Plants have evolved for millenia to grow in certain time periods-- the organelles in plants have DNA that tells them what to do--and all of this is based on the 24 hour day. You can't "train" or "teach" plants to absorb light for 24 or 36 consecutive hours...the chloroplasts aren't like a light switch that just stays on once it's on. The plant's DNA tells a plant how and when to absorb light and convert it to energy...and when not to. And all of this is based on a 24 hour day.

Could you change the plant's DNA so it would tell the chloroplasts to keep converting light energy as long as a light source is available...as in, longer than the 10-15 hours in a "normal" day? Well, yeah. But you'd need to be able to makes changes at the DNA level of the chloroplast...and we're nowhere near that. Or you could try to find a genetic mutation that does what you want, and try to cross breed it back to gain that characteristic. But I've yet to see this sort of plant mutation, so that's not really reasonable. Or you could just keep plants under artificially adjusted "days" of 48 hours with more light, and wait for nature to take its course and evolution to take place. In a few hundred thousand years or so, you will probably get what you want. Evolution of this type is a very slow process. The bottom line is that "changing" the length of the day is not going to work.

Now, changing the amounts or types of light within a 24 hour day, or working within light parameters that a plant is used to...that could do something. If you flipped a plant from 24/0 veg to 15/9 or 14/10 flower, you might get more rapid growth...the plant would, theoretically, be converting about 15-25% more light to energy. But I suspect that the DNA in most plants wouldn't buy that either, and you'd get sudden halts in growth, hermies, or both. Might be worth a try, though.