When you estimate the time it will take to the client, make sure they realize that your time STARTS when you get the data...and FINAL data at that
Your time does not start today. If your time starts today, you'll get the data in ten days
Seriously, client education is paramount. And it helps get your data together. If the client knows the clock starts when you have the data, they'll (usually) get the data to you faster. If the data changes during your work cycle, that means more time. I tend to write a contract that says changes will require a new contract, with a cancellation fee (hefty) of the current contract. They usually then decide that moving a wall 3 inches isn't worth doing it all over