Basically, the process can take as long as you want. The grand ayatollahs you see, they are linked with the Howzas for more than a lifetime now. They started out as students and, once they were able enough, started teaching in the same Howzas, while furthering their knowledge by themselves and participating in discussions with other knowledgeable people. So, after about 10-14 years, or maybe 7, people are usually well versed on the subjects and are now at a stage that they can carry on their studies themselves by reading books and don't really need a teacher. At this time, the Howzas recommend that when you feel you have studied under a teacher long enough, and can study on your own now, you rather leave studying at the Howza - so that others can come in as well ad there is no congestion. Usually, if you are a foreign student, it may also be preferable to leave the country because you may not find employment there as you wish, if every student stayed there would be problems again, and more importantly so that what you learned there can now benefit your country's people.
Of course, you don't just show up. I am Pakistani so the method I would be using is going to a Madressah, which will start teaching me the basics, such as Arabic and basics in Fiqh and after some time - 2 - 3 years - once I know the basics, people from Iran come every year to examine students and those who show promise are allowed to go to the Howza. Now, I don't really know the process for a direct admission to a Howza, which will be the case for most people who are not from the Subcontinent but, perhaps, Enlight's post can be of use there. Also, they don't usually have a lot of the formalities such as exams if you are a foreign student and go directly because there are LARGE numbers of applicants from Pakistan and, therefore, they have to limit how man can come, that's why the exams and whatever but they prioritise people from foreign countries and if, for example, you are coming from Africa, you can get in easily.
Apart from the housing, you are also given a monthly sum of money as a student, which is increased if you marry. This is because, like I have said, it is very hard to be able to work while studying and they give this money so that you are able to support yourself. It's not sooooo much but you can survive - like I have said, be prepared for a hard life.
They get the money through Khums.
I don't really know what a schema means but, as for the subjects, I really haven't gone to Howza yet so don't know fully but if I am not wrong, you can take certain subjects and leave others but there are some which are so basic, you can't leave them. Arabic and Fiqh, for example are the basics and these will be the things they teach you in the very beginning. You can't drop them. After you are good enough in these, maybe after 1 or 2 years of studying, they will open up more subjects for study, such as Rijal, which are not available from the start. If you are an English speaker, they have started madressahs, which are affiliated with the main Howzas themselves, that teach the basics, and Arabic, in English and, once your Arabic is good enough, they send you to the main Howza.
Like I have said, I have never gone to one so there is a possibility that something I have said may be wrong. Insha'Allah, ifs that is the case, those who know more about it can correct me.
Edited by Khadim uz Zahra, 16 July 2012 - 11:12 PM.