The longest biography has to be the one by Randolph Churchill and Martin Gilbert. It currently has 24 volumes right now and it is not done yet. It is likely that there are going to be more volumes that will be released in the long run.
This is so long that it has already been added in the Guinness Book of World Records. Only time will tell if another type of biography will be able to outdo this very long biography. Just to give you an idea, a lot of the biographies that you will see in the market are under 500 pages. There are some that may go beyond that but it is rare that you will see something that is beyond that.