I Plagiarised, what punishment can I expect?
I am considering turning myself in, unless the punishment would be too severe.


You are describing an extremely serious violation of most university's academic integrity policies.  The most likely punishment is that your credit for those classes would be revoked, your grades would be changed to Fs on your transcript, and you would be suspended or expelled.  (That would certainly be the most likely result at my university.)

Nevertheless, I believe you should turn yourself in.  

Ari gives a good summary of the ethical argument, but there is also a selfish practical argument.  Since you mention being caught once, there is good reason to believe that your work will face additional scrutiny.  If you get caught before turning yourself in, the result is much more likely to be permanent expulsion.  If you admit your plagiarism, the punishment will be slightly less severe; in particular, even if they decide on expulsion, the university will be much more likely to readmit you after 2-3 years.

Even if you decide not to turn yourself in, I recommend taking a short break from university, if you can afford it.  The amount of cheating you describe indicates that something is very wrong — maybe you were overwhelmed, maybe you were scared, maybe you were lazy, maybe the homework were insultingly stupid, maybe you just didn't want to be there — but something was preventing you from engaging in those classes as a responsible student.  You need to figure out what that something is and address it, or you'll just fall into the same pattern again.