First start off with a development map:
So for the planning stage, outline what you do when planning. Then associate how long it roughly takes you to work through each portion of 'planning'.
On larger projects I have spent a good 30-40 hours (or a week or so) on just planning - so that I don't have to re-do half of it and waste my time just before project soft launch.
A basic site usually requires anywhere from 4-8 hours of planning (depending on if you like or are good at it, and if the client is organized).