On our system, at around 300000 snapshots (5400rpm drive) I started to see issues with btrfs cleaning and snapshotting. I am currently testing a SSD drive instead (thanks for the help with that @Adam ), but this change to other file systems will probably lead to better use of our backup servers.
While I had 300k snapshots, that was only around 1Tb of total data. Actually uncompressed "latest" snapshots were still a lot less than 1Tb ( a bit less than 300Gb).