That would be Tortoise, as found
here.
I loved the idea, personally. The basic summary is that you have seven cards, and attempt to build a deck that goldfishes as
slowly as possible. One assumes optimal play with the deck, once you actually come to calculating the number of turns needed to win, and normal Magic rules apply, except that attempting to draw from an empty library doesn't lose. Where the opposition has choices, they are assumed to be whatever would speed the game up the most. Random effects (e.g. Mana Crypt) are best assumed to alternate between heads and tails (with Crypt, assume 3 damage every second turn, for example). Sideboards are the standard 15 cards, where applicable (e.g. Wishes).
Beyond, that are different possible 'formats', with bannings defining each one. The first such format would be banning Cumulative Upkeep and Wishes/Ring of Maruf. Additional bannings can include Upwelling, storage lands, and/or normal upkeep.
Decks requiring billions of turns are possible with Cumulative Upkeep and/or Wishes, while getting into the thousands is a good effort for decks that avoid those options.
