Well, it depends. If I already told them I would do it then personally I would feel sort of obligated to. (That's just how I try to be).
Of course, if you are still hashing out the requirements you probably haven't committed yourself to the project because you don't know what it is yet. If it's more than you have time for then you may have to just tell them, 'Wow, this is more than I expected and I have school starting up in a bit. I don't think I will have time to complete this for you.'
On the other hand, if you make brushes for all that stuff you might be able to crank it out pretty quick.
edit: On a side note, someone once said you should always get something for your commissions. Even A dollar for example. This allows you to put them down as a paid commission which if you ever plan to do this type of work on the side could be of value. That said, I have done them for free in the past.