This completely depends on the number people using it and number of times it has been used. If you are using it regularly the you should drain it monthly or if your are using 2-3 time a week then drain it after 2-3 months.
You can refer this video for replacing the Garbage disposal, this has helped me a lot.
That would look really cool, you can experiment a lot with colors like purple to give a creative look to your interior.
I think you use white, light yellow, brown or some bold color like red, Pink and all to get a contrast look.