Your agent just called you, and you have an offer on your home. When should you get back to the buyers with a yes, no or a counteroffer? As quickly as you reasonably can, period. There is nothing worse for a buyer who has placed an offer on your home to have to wait DAYS for you to get back to them, especially when there is no excuse for it.
Have you ever heard the expression to "strike while the iron is hot"? If you have good reason to wait to reply, let your agent know, so they can inform the other agent that they will have an answer on such and such a time, and stick to it! Now I understand that you may want to, or have to call your attorney before you respond, just communicate that. That's all. Let your agent know. The lack of a quick repsonse indicates disinterest to the buyer. All of my clients have been wonderful, but it has always bugged me to no extreme how some agents (and attorneys, btw) will not return your call if they have nothing to report.
I just wasn't brought up that way, and a phone call to say, "Hey, I'm thinking of you. Just wanted to let you know that I haven't heard anything yet" goes a long way with me, and I always keep my clients up to date, or communicate with them so they are not left hanging.
Just to reiterate, no one is saying you must give an answer in 5 minutes- just don't wait days if you don't have to. You might want to respond in "strategic time", but that's a conversation that you and your agent will have had already, (or should have). One of the best rules of negotiating is to put yourself in the other's place, and imagine what they would think of your reply. The goal is to make your negotiations a win-win. So when you need a top notch negotiator, call me. I'll negotiate for you, like I would for me, and my family :)