I would approach this question from a couple of view points. One like having a custom house built, If you make changes, After the work is started, You will add time to the work, and expense.
Two, a custom built street rod, Nothing is standard off the shelf, that is the way behind "custom". If you have one guy doing the work for you, or a crew of ten.
Both could take the same amount of time. Some one here hired a neighbor fellow that was very very good with wood, The time he took and the level of fit and finish, I would call this cat "master" wood worker. There are damn few "master" of anything. The closest I can think of are people that outfit boats, planes, and way way expensive tour/coach buses. School bus, crew of five to seven, making 30 to 50 an hour four weeks or less.
Most here are amateurs and many do very good work. Because this is not our every day thing 8 to 12 hours a day... we do an hour here five hours there..... takes a year or more.
sorry I can not answer your question, but with rare exceptions, any one that can say it will take x number of days, is lying to you.
Only someone that is doing this every day is the most likely to be able to answer this question. and even then, it goes back to that thing about making any changes after you start.