SR Tracker will track for how long your players been playing on your server with the ability to use mysql or sqlite.
Edit sr_tracker/lua/sr_tracker/sv_mysql.lua to enable mysql
thelastpenguin™ for https://github.com/thelastpenguin/gLUA-Library/tree/master/pON
Alexander Grist-Hucker for https://github.com/alexgrist/GLua-MySQL-Wrapper