they can leave whenever they want, as with any fly ball, and if the ball drops, they may advance, otherwise they need to return to their base.
The infield fly call simply removes the force play situation, so the runners can stay at their base even if the ball drops.
That is the idea of the infield fly rule, to protect the runners from easy double/triple plays by letting an easily catchable fly ball in the infield drop.
See OBR definition of terms/infield fly regarding this.
Hope this helps!
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