I think there are 2 major examples of market sizing cases: (1) doing a small market sizing inside a bigger case and (2) doing a case interview solely on market sizing alone. The second case is a small fraction of cases but certainly a possibility (between major firms Oliver Wyman is the most known to do it).
Let's start with example number 1 - since this is just a small step inside the case it's usually something that you can have a grasp pretty quickly - I would so between 30 to 60 seconds is definitely within reason; If it's something really challenging and you are not getting there, sharing your doubts with the interviewer and talking through the problem would give you some time.
The second example is something a little more challenging if a case revolves around just a complex market sizing problem. I would say the ideal time is similar to the time you would take to do a full structure at the beginning of a case which I believe shouldn't go over 2 minutes. I've given full market sizing problems from real interviews and the interviewee took 3 minutes to come up with a strategy to solve the problem and it really felt like a long time in full silence looking to a person thinking about out to approach a problem.
my 2cents, hope it helps!