Gaining control lasts for as long as the modifier specifies. If the card says for 1 turn, then it lasts one turn. If it says until the card used to gain control leaves play, then you control it until that card leaves play. If it as attachment that gains control of the ally, then you control the ally until the attachment is destroyed.
Indefinite times are until the ally is destroyed, like allies taken via Ishanah's effect.
Basically, go by the card.
Level 0 Judge (Because apparently under 16 year olds can't get Judge certificates =/)
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