There are no time limits as far as free slots are concerned. These are not like the normal casinos since they are hosted on servers. Since there is no manpower required to take care of these online virtual casinos, the free slots are open throughout the year. You can join the online casinos where they are hosted whenever you like.
Just turn on your PC and connected to the internet. Fire your browser and log on to the site that offers free slots. Once you have done that, you can start playing your favorite games. Play as long and as much as you like and you will not be charged a single penny since free slots are really free.