See the following link for historical max elevations back until 1982 - June 3 maxed out at 5.89 on the gauge; I believe that was 1984 or 1985 -I have some photos (slides) of it somewhere.
http://des.nh.gov/organization/divis.../chart2009.pdf
FEMA list the 100 year flood elevation of Winnipesaukee as 506.
At the current lake elevation and discharge rate of 1,888 cfs it would take about 9 days to get down to a lake level of 504.32 - IF THERE WERE NO MORE RAIN AND NO MORE WATER RUNNING INTO THE LAKE