Loops

How do I tell the game I want to execute a loop of a sequence of game actions X number of times?
Not sure how current rules work but it used to be not just possible but in fact required to do so and define a limited amount of X to make sure infinite loops don't slow the game.
Right now I'm playing a Palinchron Mana Flare deck and tapping/untapping all my lands as many times as the enemy has life total is...unrealistic to say the least. If I did this is a real game I'd actually be breaking the rules by stalling and would be forced to say "I want to do this 19 more times" instead without actually doing it.
Not sure how current rules work but it used to be not just possible but in fact required to do so and define a limited amount of X to make sure infinite loops don't slow the game.
Right now I'm playing a Palinchron Mana Flare deck and tapping/untapping all my lands as many times as the enemy has life total is...unrealistic to say the least. If I did this is a real game I'd actually be breaking the rules by stalling and would be forced to say "I want to do this 19 more times" instead without actually doing it.