A very rough estimate: assuming you have 10 experienced developers on the team, each can read and comprehend 1000 lines of code per hour. Given a 10-hour workday, the team can digest 100000 lines of code per day. To finish just reviewing the code, it will take 1000 days, about 2 years and 8 months. Not sure how much time you have and when you would expect to deliver the final product. On the other hand, if you can find out the use cases and even take a look at the current product, you then may not have to review the source code but just go ahead implementing the features.