According to your prospect, NEVER. It doesn’t matter what answer they give you, it puts you in a very defensive position. Their timeline is not yours. They might say “maybe call me back in a month, I am very busy right now” which means you did not properly qualify them on the phone. You didn’t get a commitment and they will take that extra time to dismiss you because they didn’t properly erase all their objections in their head which you could have helped them overcome to make a decision. You want to find out what’s going on and get precise. If they say “call me back,” set a specific time; but, also provide to that prospect information that can help them make a decision to move forward before your call. When you have to chase them it gets you in a defensive/passive mode where you have to say “well, I am just checking on you and seeing where you stand”.