A clear and concise title.
A draft costs you 1500 gems. You gain more gems by winning enough matches.
You can keep playing until you’ve either won 5 matches or lost two. To win your gems back (and some packs, but you can’t use packs to draft, so that’s not part of going infinite), you need to win 3, so it would seem that you need a win percentage of 60% to go infinite, but it’s not that simple.