How far away can you be to play MCPE with someone?

According to this wikihow:

Both the Android and iOS versions can play online for free, without the need to install other apps or jailbreak your device

So to answer your question, you can play with anyone regardless of how far away they are, as long as you both have a connection to the internet.

The only thing that the physical distance (from the server) would affect is the ping. However since data travels at the speed of light for most of it's journey the ping should still be at playable levels regardless of location.

Steps to play on the internet, for more detail see the wikihow or this video tutorial

1. Update Minecraft PE
2. Find the server IP from your browser by searching for Minecraft PE Servers. (Listforge is a common one)
3. On your device, open up Minecraft PE, and click play
4. Click the button with the plus sign and the arrow to the left of the advanced button. This is the Add External Server
5. Enter any name you wish, the IP or Address and port must be the same as what is listed in the 2nd step.
6. Click Add Server
7. Click Back and the server should appear in the list and you can now join the server.