Time Machine backup time
After searching, I've found that generally, even though this is the stated timing 



the best you are likely to see is every 2-3 hours for this size drive. Sometimes a backup will stall, and the software will restart, so I have a few daily backups remaining that time stamp as late as 6 am, showing there was at least a 6 hour gap. 

That said, I have a second MacPro, running Yosemite, same 50GB used, and its backups are every hour, like clockwork.