There is no set limit. However, server configurations determine how much can be backed up. Script timeout time, memory limits, CPU usage, and hard drive utilization all can have an influence on backup capabilities. BackupBuddy is meant to back up WordPress sites, not entire servers or vast amounts of data when used on shared hosting. Shared hosting usually allows for backing up sites 400-1500MB before server limitations prevent progression. This is simply a limitation of your shared hosting environment - like the name states, you are sharing resources with dozens, if not hundreds of other users on each server. If you have your own dedicated server you can adjust settings to increase its capabilities.