If I sit and meditate for 15 minutes, is it less effective than sitting for 1 hour or longer?

It depends how long you have been practicing meditation for. If you are new to it, sitting for long periods of time, even for 30 minutes, can be too much. You might just not be used to it, you might not be sitting correctly, you might experience more back and knee pain, and a few other things that could cause you to burn out quickly from the practice and just stop it.

In comparison, meditating for even just 5 to 10 minutes a day in the beginning has quite a few benefits, the main one is that you don't burn out and get tired of the practice so quickly, instead, you build it up and make it a long term practice. Other than that, it helps build stamina, strengthens the muscles required for sitting, and allows you to slowly get into the mindfulness part of the practice with less stress.

So I would say that sitting for 15 minutes, or even 5 to 10 minutes can be more effective than sitting for 30 minutes, 1 hour, or more. Even for the more advanced and well practiced meditators, sitting for such long periods of time is not necessarily more beneficial than a short session.