The honest answer is that the headhunter will either be all over you like a cheap suit or will more than likely never get back to you. The headhunter will chase you hard if he/she (to keep it simple will use he/his from now on) thinks you are the answer to his current challenge, i.e. helping him fill the client vacancy he is working on, either as a candidate or as a source for finding the right candidate. Either way, you won’t need to ask this question because the headhunter will chase you hard.
If you need to ask the question, the answer will probably be never!
Headhunters work for clients not candidates and unless you are the answer to his current challenge, you won’t get a call back. That’s why I tell people if you want to get on the radar of headhunters, be a source of candidates if the role isn’t right for you or you are not right for the role. Headhunters are far more likely to build a relationship with you if you can help them by being a source to find others. If not, you’re unlikely to get their attention for very long because the chances of you being a great match for their next assignment is so slim that it makes no sense for them to build a relationship with you. This may sound harsh but when good headhunters or their researchers are searching for an open position, they may make 70+ contacts per day and it would be impossible for them to build relationships with all of those people. In fact, they are trying to qualify very quickly if you are a possible candidate and if not, they need to move on (unless you can help them find someone else who is suitable of course).  The exception is when a headhunter works in a niche market and is likely to have several more assignments in the same sector, so even if you are not right then, he may keep dialogue open and park you for future assignments. In this case, the next time you hear from the headhunter will probably be if he thinks you are a possible match for his next assignment or that you might know someone who is.
The confusion comes because people think headhunters work like recruitment agents. True headhunters do not, they only work for clients. Agents rely on a stream of vacancies from employers (usually not exclusive to the agent) and a steady stream of candidates to fill vacancies. They need a steady stream of both to match and make placements so agents are more likely to get back to you. You might be a match for several of their current vacancies or they can see you being a match for future vacancies, e.g. a company they know that takes people on with your type of skills. The headhunter has a completely different focus: finding the exact match for the vacancy that his client has assigned to him.
So if you are waiting to hear back from a headhunter, don’t hold your breath!
