Let it rest as long as you can, if you rush this it'll ripple. A lot depends on how long this has sat in it's tube and how thick the paper is. You may never get the print as flat as you want it, in this case a window mount will help when framing. good luck!
has anyone here ever used poster mountain’s (postermountain.com) flattening service? i’ve heard they do amazing work. it doesn’t come cheap though plus you have to pay for shipping both ways, and on the return trip it’s shipped flat.