Ok. 2 Questions:
1. What is the maximum minus (debt) an account can have?
2. Is there a mechanism that will force you to at some point balance your accounts debt (go to 0)

You can see Circles as such a mutual credit system as well. In this system question one is answered with: that is the basic income — the number of coins that you have issued. Lets say you have 0 units eco but you are allowed to go to -1000. In circles we would just say your account is at 1000. It is basically just a naming convention at what point to use the 0 as long as the answer for question 2 is: basically there is NO mechanism (like interest rate).