First off, I know it's just a trade server, but at some points in the day, there are quite a few people who come along and go idle for quite some period of time, and it interferes with people getting into the server to play around or do some serious trading.
I might always be able to get in because donator stuffs, but you always have to think about the non-donator player base for the servers as well.
Some people might be standing still while doing trades, but even then that's only for a few minutes, so I would suggest a 15-30 minute kicker? That sounds pretty fair IMO. The trader server is extremely busy in the middle of the day and after the youngins get out of school.