When can I start walking away from a raid?

In my experience, once you see "Go" you can walk away. However, if you experience some sort of network error then you'll be booted out and have to return to the original location if you want to resume where you left off.

I don't know the exact specifics, however the general idea below should be accurate.

There's a range to enter the lobby, say X.
There's a range once you've entered the lobby to not get booted, say Y.
Y > X.

Basically what this means is once you're in the lobby you can walk away a bit (e.g. into a nearby building that can't quite reach the gym to enter the lobby) without getting kicked. E.g. there's a gym near my house, I have to walk about 20-30 meters to get in range to enter the lobby, but I can go back home and do the raid from inside.

Once the raid is finished and you're on the catch Pokemon screen, you can go away as far as you want. You can tap the screen to get through the reward and ball interfaces quickly.

Note that if your connection drops, you need to go back to the raid to re encounter the Pokemon. If the raid has timed out you'll be unable to get it. So walking away too far may be an issue if you walk through a dead zone or maybe lose/gain wifi, etc...

You can start walking away as soon as the raid starts if you know you won't need to rejoin and if you trust that your game doesn't crash. It's a bit of a risk, but I do it a lot when I'm in a hurry and it has rarely backfired on me.

The game only checks for proximity when trying to enter the raid lobby and when trying to enter the fight from the lobby. You need normal gym range to enter the lobby and an extended, about 80m range, to enter the fight.

Note that with the current update there has been a lot of server timeouts where you can get booted. Also, previously if your team needed to relobby, although you would not be able to do and could just wait till others beat the raid boss, now you may get kicked out at the end

It used to be that you could walk away once the raid actually began - the Raid Boss showed up, the Go! came up and you could fight it.

Lately though, I've been kicked out of 3 Shinx raids when I tried to do that, and had to stay in the gym range until I beat the boss. I haven't tried with any other raid bosses (I'd like to, but I'm not going to waste a raid pass on a Buizel raid or try to solo a legendary just to test this out. Seriously, the last two days all I've seen are Shinx raids, Buizel raids and Legendary raids. RNG, man, it's a b****) but it's been rather annoying. So I don't know if this particular issue has been "patched" or not, but I would be wary of doing it on raids when there isn't enough time to relobby if the game doesn't let you use this trick.

(which cost me a Shinx when I hopped into a raid while at a drive thru, and by the time I got my breakfast, the raid had expired. ARGH.)

If you disable GPS you can walk away as soon as you enter the lobby, but won't be able to rejoin.