Decision time on a postdoc offer while applying for other positions
Most likely, you won't be able to "make" them wait at all.  If they give you an offer, they will also give you a deadline to decide whether to accept it.  You can ask for an extension of this deadline, but there are no guarantees that they will agree to extend it.  The power here is mostly on their side, not yours, unless you are really a superstar, or are the one and only qualified candidate.

For a position like a postdoc, the employer typically has a fixed number of positions available (often just one).  Commonly (especially for a field with an academic-year hiring cycle) they interview a number of people and then make offers made serially.  Every day that they wait for you to make a decision is a day in which the next person on their list might take another job, reducing their options if you turn them down.  So it's not in their interest to give you an indefinite amount of time to decide, since that could result in them hiring someone not as good, or not being able to fill the position at all.  They don't want to rush you into declining, but they also can't wait forever.

It's hard to predict what kind of deadline they might give you.  Some employers will give a candidate a very short deadline (a few days) if the candidate is a "reach" - the employer doubts that the candidate will accept, but wants to at least try to get them.  This way, in the likely case that the candidate says no, not much time has been lost.  More commonly, the deadline would be on the order of a week or two.  Having as much time as a month seems unlikely, but there are a lot of factors that could affect it, and if they are not tied to the academic year there may be more flexibility.

What "makes sense" to you from a personal perspective (pondering an overseas move, exploring options, etc) is not really the employer's concern.  Even before receiving the offer, it would be a good idea to carefully consider all the pros and cons, and come to a tentative decision about whether you would take the job if offered (including scenarios about other offers received or not received).  Between the interview and your own research into the institution, you should have almost all the information you're going to get; the only extra information in the offer itself would be the salary (and other benefits).  So ideally, as soon as you get the offer, you should know how you intend to decide, or what other offers you need to wait for.

It's entirely possible that you may have to make a decision on this position before hearing about the others.  That's just how it goes.