When you have cooking oil in your fryer the fryer oil should last up to 3 to 6 months depending on how often you fry foods.
I only deep fry foods every once in awhile so my fryer oil lasts me at least 6 months before I need to change it out.
If you notice your fryer oil getting darker in color then it would be a good idea to change it out.
When I need to dispose of cooking oil I just let it cool down and dump it out on the ground which is perfectly okay as the cooking oil is not like car oil where it would harm the ground and groundwater.
So you can either dump the fryer oil into a container after it cools down and throw it in the trash can or just dump it out on the ground.