Let me start this off with a question for you: do you already know how to lose weight, from past experience?
If the answer is yes, and somehow you have forgotten how it went, but you do know what it takes, and are ready to start implementing your plan immediately, then I would say a good number you should be able to hit is 4 lbs per month without having to be too extreme.
That being said, if you have absolutely no idea where to get started, then it will definitely vary, depending on a bunch of factors.
The main factor here is going to be your diet, and your caloric intake. You may have heard this before, and it is definitely true: that a six pack starts in the kitchen. (There could be an exception to this rule, where someone exercises a ton, but for most people, this is the case, because you just typically can’t out exercise a bad diet).
Otherwise, if you are starting from scratch, then you will need a plan, because this is going on for 30 days, and you will need to stick with it for 30 days. In this case, it all depends how soon you adjust your calories, and then how much activity you add into your week.
The basic formula for weight loss is that you need to create a caloric deficit of 3,500 calories for every pound you will lose. In my experience, it worked out pretty much exactly like that. Different people with different body types will get varying results, but that does seem to be a good rule of thumb to go by.
Then on the other end, for someone more extreme, looking to really go for broke, some people have claimed up to 10 lbs a month lost, or even more. That is a more extreme case, where they pretty much have to exercise quite a bit, or are cutting way down on their food, which may not be the healthiest choice, but in order to lose 10 lbs in one month, you’d have to create a caloric deficit of around 35,000 calories in that time, or a bit under 1,000 calories per day, which is almost half of the average calories you’d be consuming daily.
In this case, you should talk with your physician to make sure any of it doesn’t do more harm than good.