how do i know when my new tank is ready for fish to be added? how long on average does it take (i have some old filter media in there to speed things up)

Do you have fish in there or are you adding a source of ammonia? you need something to keep the bacteria alive from the filter media.
To tell when the tank is ready for stocking properly you need testkits for ammonia and nitrite. When these are at 0 you can slowly stock your tank.