There are two reasonable ways to do this, and they depend on the situation you face. One is:
Hi Boss, you know that MBA I've been working on? It's time for me to schedule the XYZ part of it, and that's going to require two months of my solid attention. I'll need to take an unpaid leave from here. I can do it any time in the next 8 months or so. When can we sit down and choose when to have that time off so it's easiest for you?
(Ideally your boss has known for years that the XYZ part was coming, and is in general in favour of your MBA.)
Hi Boss, you know that MBA I've been working on? It's getting too difficult for me to work full time and get the MBA finished. I have decided the only way to get it done is to take leave from here and go heads down till it's finished. I know that seems extreme, but I'm just not making progress in the evenings and weekends. I think I need about two months. How can we plan this so I get on it as soon as possible, but you're not in the lurch here?
An awful one is:
Hi Boss, I've known for years that I needed two months off, but I never mentioned it to you. This need for leave cannot be postponed or adjusted in any way. I need to be away from this date to that date, and I can't budge on that. Can I have the approval for the leave please?
It's probably best, before you ask, to answer the following questions for yourself:
- if you are denied the leave, would you quit? If so, granting you leave is something your boss wants to do so that you will come back afterwards. If not, getting the leave is something you want from your boss. What are you offering to motivate that?
- how long have you known you would need this leave? Have you just realized it? Is that because you're disorganized, or because something has changed in your program?
- how firm are your dates? Can you do a one week on, one week off thing for four months? Can you just lower your workload to two days a week? Do you need to physically leave town?
- What would happen to your MBA if you didn't get this leave? What are you willing to do to prevent that?
If your boss wants to help you, and you're willing to work to minimize the crisis effect of you being suddenly gone, this can work out. If you knew, didn't plan, didn't ever alert the boss this day was coming, and have no flexibility to make it work, then chances are you and your brand new MBA will be looking for a job two months from now. That's not necessarily a bad thing, just be aware of it.