As long as you want it to be.
It’s your meeting, so we let you decide! You have the poll controls, which means you can keep it open for as long as you like.
We've had users close polls after 5 seconds, we've had users close polls after several hours. If you're thinking about anything longer, then it might just be worth contacting us.
If you're using the PowerPoint Add-In then you can add a countdown clock to your polling slides. This is a great way of letting your participants know how long they've got to submit their response (and it adds a nice consistent feel to polling).