The rules are more or less decided by the one that is sharing some of his/her win: if you read through the thread, you'll see that most people set the rule to give away the same amount as they have shared, as soon as their gift has been doubled in the Casino, by the lucky player...
It's all in good faith of course, and there's no minimum or maximum
Other rules that are pretty common by now is that the participants for the giveaways are more or less established members, with a decent post count etc., to prevent newbies or otherwise inactive players joining in to share in the fun, then disappear 9or similar scenario's

)
And WTG winners of late! good to see the thread still thriving in 2018 -and on that note: Happy New Year my generous friends!