[dmos62]

I use Breathly. It's great visually and audibly. And, you can use a custom timing. It also has a few other presets, including box breathing. And, it keeps the screen on, though I wish it would work when my android is locked (in my pocket).

https://github.com/mmazzarolo/breathly-app

[cjauvin]

Great app, just tried it, works very nicely and has just the right set of things it should have, thanks!

[dartharva]

There's also Medito (closed-source but nonprofit), which has all sorts of guided meditations.