Keeping still while her ribbon dries with 〜間

Grammar Explanation: During the same time with 〜間(に)

Kyouko’s ribbon needs to be washed. Since she is a jiāngshī (vampire), she will uncontrollably attack people to drink their blood when her ribbon is removed. For this reason, her brother tries to tie her up with a rope. Kyouko hides in the bathroom, then sneaks out the window only to find Kenji waiting outside.

ケンジ:: 「干(ほ)してる間(あいだ)じっとしてればすぐ済(す)むだろ。」; “While it's drying, if you'd just keep still it'd be over soon.”
ケンジ:: 「なんで毎回(まいかい)そんなにイヤがる！！」; “Why do you seem to hate it so much every time?!”

Key Points

1干してる間 means while it is drying

干(ほ)してる is the casual form of 干(ほ)している.
間(あいだ) marks the time span during which the ribbon is drying.

2The condition is じっとしてれば

じっとしていれば means “if you stay still”.
してれば is a casual contraction of していれば.

3すぐ済むだろ minimizes the problem

済(す)む means the matter will be finished.
だろ frames it as obvious: if Kyouko waits, it will be over soon.