One of my golden rules in assessing how much time it will take to do a job :~
a. Make a reasonable prediction of the time you think it will take
b. multiply that time by a factor of 2.5
c. Take another look at the job in hand & ask yourself if 2.5 will be enough (very pertinent if you are trying something new / experimental / untried / for the first time )
d. Build the result & cost into a fee proposal.
I can accept no responsibility for any inaccuracies you may experience but I find as a general rule it is far better than "a." on its own