Requesting AWS limit increase of EC2 instances - how long does it take?

I'm going to say it depends on your support level. Assuming the basic level, it's 24 hours I believe. If you are an enterprise you can expedite by going to your account team (which they now do have for larger customers or opportunities).

Just do document my experience: it took Amazon 17 hours just to acknowledge my request, which they escalated to the Service Team so it will take another 24/48 hours.