Your mind is an instrument, so why use it 24/7? At the end of the day, it's just like your arm, or your foot, it's like a muscle. And you don't use your arm all day, nor do you use your foot all day. So why use your mind all day? Doesn't make sense. Like you allow your foot, and your arm to rest, you should also let your mind rest.
That's precisely the reason why you should meditate.
Now, I have recently realized that timing your meditation sessions just doesn't make sense. Because a lot of people make a goal of it to meditate a certain amount of time. They put an alarm on 10 minutes for example. But the thing is that, because of that, it becomes a goal for them to meditate that long. It's no more about meditating, it's about reaching that goal. To be honest, I don't even know how long I meditate every day. I really don't. Because it just doesn't matter. What matters, is that I meditate.