what percentage of a banks money do they lend
I'll put my comment as an answer and add one or two things. To see how much a bank lends in comparison to deposits, you can look at what is called the loan-to-deposit (LTD)ratio of banks in their annual reports. Usually it is between 60% and 90% but as you mentioned this can vary upon institutions .. And geographic areas as well ! For instance, in 2015 in the EU the average LTD ratio was 108% whereas in the US the average is about 79%. You can check per state the LTD ratio from data gathered by the Fed and the FDIC. 

As to factors that determine this amount, I would probably put regulation as the first point. If I had to think about other macro points that could affect the LTD I would probably mention interest rate and in particular LIBOR rates as they tend to give an idea about the financial stress on the banking level and therefore to a certain extend the willingness for a bank to lend money and/or to require more liabilities.