When can you Meditate?
This question is simple to answer. You can meditate at anytime that is comfortable for you. This could be in the morning, afternoon, or evening. You can meditate once a day, or twice a day, or more if you are up for it.
Some websites that you should meditate around the same time everyday so that it becomes routine. If meditation doesn't become boring because of this, go right ahead and do it at the same time everyday. The problem with using the word "routine" is that is may make meditating boring, and just another thing to do on your to-do list. For me, I meditate whenever I feel like to, and therefore at random times. It makes meditating less boring, and less like a chore. But do whatever is best for you.
Now when you sit to meditate, how long should you meditate? As long as you want! For beginners, they may not be able to last for more than 15 minutes, whether its because they get bored, or uncomfortable, or too distracted. That is fine, in time meditating will become less boring, and more relaxing. If you are just starting out, start meditating for about 10-15 minutes, and then slowly add time as you become more comfortable.
Its fine if you don't meditate for hours at a time. 15 minutes a day is enough to notice a calmer self.
So in conclusion, meditate anytime you want, and for however long you want. It is all up to you!