Logging MAC addresses using condition

would like to write a log file with MAC Addresses for all clients which connection last more than 15min (uptime).
Please help, can't find any solution for this problem.

I do not think such tool exists right now, I guess you will need to write it yourself. I would start by having a look to the logs, looking for relevant messages when clients connect and disconnecting, then write a script that waits for those messages.