You get a response instantly, within a minute or so. I would be careful with Priceline for flights because there is too much risk, too little reward. You used to be able to get great deals on flights but now the savings are minimal and for that you have to be very flexible and be willing to take a flight that may not be desirable--at least when you use the bidding option. The flight can start anytime from 6a.m. to 10 p.m. on the day of travel and though they "try" to find non-stops first, there can be up to one stop. Don't plan on being on the hill that day with those caveats.
PL works great for hotels if you use biddingfortravel.com or betterbidding.com. Poke around on those sights and you'll see how to get free re-bids and save on hotels. BTW, the feedback for flights is almost non-existent on those boards now because not many people use PL for flights anymore.