You will not find anywhere a fixed number of days for any of these categories. I had incidents entered with low priority and got them solved and closed in just a day. I had medium categorized incidents that took more than 4 weeks to get the first time opened, and still one more week to get a reply.
If you have hundreds of incidents in the same group, then the new incident is just added at the end of the queue and has to wait until all others are taken care.