There is no set or average number of points your credit score will improve each month when you take out a credit building loan. When rebuilding credit, the path to improvement depends on the reason your credit score is low:
If your credit score is low because of poor payment history and/or debt collections, then it will take months of on-time payments to see any improvement and a few years or more to reestablish good credit.
If your credit score was negatively impacted by a high credit utilization ratio, and you pay off your balances in full, credit repair can happen quite quickly.