Not long ago I went through the details of the Flood account in Genesis to see how long it lasted. The total time from when the rain started until Noah exited the ark was 1 year and 10 days. The total number of days depends on how many days were in each month, and there is no authoritative source that can define with 100% certainty how months were measured in Noah’s day (or Moses’ day, since that’s when Genesis was written).
Assuming 30 days in each month, here is a table that shows each event described and the accumulation of days:
For a rough visual indication of the rise and ebb of the flood waters, see this diagram:
originally posted 3/22/2005 on bibleforums.org