Meditating in the shower is pretty easy IMHO and you can easily go up to 15 min, even for beginners. Meditation is mostly about anchoring your mind to something, most people do it with their breathing because that's what we see in movies and stuff.

In the shower, you can use the sound of the water or the feeling of the water on your head to anchor your mind. Since it's something you don't control and there are multiple sensory reactions created from it, it's pretty easy to be present and observe your thoughts.

This doesn't necessarily work for everyone but I have tried many types of meditations and this one is the easiest for me.