It seems to me that if 4 - 6 hours was quoted, that could only have been over a period of 9 to 24 months (as one doesn’t know how often the client will show up), and only as a case of “at worst, 4 to 6 hours total spread out over time” and even then only because one might be talking to someone one has never seen.
It is intersting how some people expect to call someone up on the phone and ask, "How much and how long to paint my house, and the person they are talking to has never seen the house, and has no measurments to go by.