I play the most on Wednesday and Saturday, other than that I mostly just log in. I still can make 30k+ cp on these two days only but it's cost a few refills. It's easier if you finished all the explorations. There are a few days which I suggest to farm more or to play on those days ignoring exceptional;
1. Tuesday or Fire type bonus. Do Exploration Episode 2 1-1-2 [Entrance of Forest of Confession] 4hr
2. Wednesday or Frozen type bonus. Do Exploration Episode 2 1-4-3 [Cafeteria] 8hr
3. Saturday or Darkness type bonus. Do Exploration Episode 2 1-3-3 [Gym Closet] 6hr
You may also want to do the last Magic class.If you are used to last minute farming and know how to pull the tricks, you can get around 40k in two hours on Saturday but be warned, it could end up costly.
Other days if you are daily active player;
1. Monday or Normal type bonus. Do Exploration Episode 2 1-1-3 [Hill of True Heart] 6hr
Monday give about the same cp as on Tuesdays. Most people need to work on Monday and got a bit busy on that day, so I put it here.
2. Thursday or Thunder type bonus. Do Exploration Episode 2 1-2-2 [Liberal Arts Classroom] 6hr. About same cp as Tuesday and Monday.
Relax day?
1. Friday or Holy type bonus. There aren't much cp to get on this days but Exploration Episode 2 1-5-5 [Student Body Office] gives a good amount of Holy type enemies for circle points.
So now for the Mileage points. As what most people said, get active friends. You can get lots of Mileage points by defeating exceptional.I can give you calculations and a few examples if you need it, putting everything in a wall of text is not really my thing.
Now, next to active friend is for you to see your best team, how far you think they can go? If your team is not good and go away easily, you will have to farm more. Having a low level exceptional can help you with that. Having a low level exceptional can be disadvantage if your team is really strong since making the exceptional to high level is taking time.
Well, then good luck and have fun. If you need anything more about the topic, don't hold back to ask.