I have been doing part time job plus detailing for 5 years. This year I might go full time detailing.
My typical week is 2 to 3 days at my regular job, and 2 to 3 days detailing. The days I am detailing as very physical, I am mobile and work alone so on most detail that is a 8 to 14 hours day for me with almost no breaks.
This year I decided to hire someone to give me a hand. If it works out, I may go full time.
What you have to remember is that detailing is very taxing on your body. I don't think you could work 40 hours plus 2 days of detailing for very long unless you are an athlete and are in peak physical condition. When I first started detailing it would take me a day of rest to recover from the detailing. Now I am used to it and I merelly feel tired or sore on the next day but still can fonction normally. I do like to have a day off between details if I can. With help that will likelly be much less taxing.
So my advice, if you can, ease into it. See if you can cut 1 or 2 days from your normal job and add 1 or 2 days detailing at first. As demand grows you can cut an other day at your work to do detailing and so on. Not all jobs allows for this but if you can make it work with yuor employer, that is the best way to do it IMO. Not only do you have more financial security this way, but you don't have the stress of accepting any detailing job that comes your way. It gives you the option of choosing what detailing jobs you want to do.