Most people that develop for selenium, and are familiar with setting up selenium grid servers are familiar with VisGrid. It is a gui, to help you manage the hub url, and what drivers you want to maintain. Right of of the box this software has default settings that will close your selenium session for inactivity. This could be bad depending on the script you are writing. Start it up like this and it should solve your problem
This will allow the time limit to be unlimited and you should be able to use your selenium session for as long as you like.