By counting your breath, you can figure out different patterns of your own breathing. Mentioned above, the "objectively, optimal" breathing rhythem would be 5.5 seconds inhale and 5.5 seconds exhale. But this is just a statistical number, and might not be the right pattern for you.
So, let's look for your optimal breathing rhythem. Get into a good position first. Lying down (if you don't tend to fall asleep or you are not tired) or sitting up in a good position with your pelvic floor parallel to your diaphragm and then let your breath flow naturally. Once you feel like your breathe has settled in, start counting your breath. It's not important how fast or how slow you breathe. Just count and keep it kind of consistent. First, you just keep breathing as you are and count, no matter how the pattern is, then slowly start to make the inhales just as long as the exhales, and then try to keep going in this pattern. Focus on breath quality, using all the right muscles.
Breathing for Relaxation
If you want to slow it down, want to get ready to sleep or just need to relax, you can change your counting accordingly. Start like you did with the symmetrical breath and after a couple of rounds of symmetrical breathing lengthen your exhale. If you were breathing 4 seconds in, 4 seconds out, lengthen it to 5 seconds out, then maybe try 6, 7, or even 8. Test your own comfort levels but don't force it.
Another alternative for a calming and relaxing breath is the box breathing. With this exercise you focus on the pauses between your inhales and exhales. For example you can inhale for 4, hold for 2, exhale for 4, hold for 2. That would be a good beginner's pattern. If you feel comfortable with that, try to go for 4:4:4:4. As with all the practices, don't force it. Only go as far as you are comfortable.