It's basically instant if you buy it online, and near-as instant if you buy it at a hotel (though there is no real reason to do so unless you're at an ibis budget anyway - unless you want to pay cash I guess).

If you are cancelling and rebooking often, worth noting that the ibis business card gives a guaranteed room if booked 48 hours before, vs. three days before for ALL Gold (though of course the ibis card is only for ibis properties).