Having said that, from personal experience I'd say 20 minutes early in the morning is best. I know people who meditate for an hour every day though so there really isn't an upper limit to meditation. But always remember to start small and work your way up.
I hope this helps 🙂
There are many meditation apps, and most have some free content, to help with guided meditation.
Once you get the hang of it, you can combine "going for a walk" and "meditation" by walking mindfully. Google it for ideas!