How far is too far? If you are doing an assessment, maybe you could go back a few more times for your peace of mind and the client’s.
I charge a monthly rate for email and/or telephonic coaching, which comes to about $10 a month. I like Aaron’s idea of breaking down the hourly rate to four messages; so, that is what I will charge starting 2013: four messages/answers per hourly rate.
Hope this helps you, good luck.