Frequently enough that they know what "Get Help" means straight away even though they're thousands of years old, if they only done it once in their long lives they presumably wouldn't remember it, so it must've happened a fair few times.

Before the events of Thor, Thor and Loki would fight together in many battles. I don't think that move would work in heated battles, so I presume it was for more of a infiltrate or stealth missions like we saw in Ragnarok when they were trying to escape Sakaar.

It's probably something they did as kids to other friends and stuff like that

Lol definitely enough to have had a nickname for it.