[raker]

If there is a truly unstoppable loop (considering usage of the word "may" or such optional ability to continue or break a loop) then the game is considered a tie. There are tournament rules regarding players intentionally drawing out the game (it's against the rules), but I don't think there's any rule against a deck that simply runs long (or really long).

[thaumasiotes]

Some cards have been banned for causing tournament rounds to run too long, or for being central in decks that regularly do this (I believe Sensei's Divining Top and the "eggs" deck were the major offenders).