The clocks are changed in a seven month period, from April to October and in a five month period from October to April.
It actually comes out to a little under seven months and a little over five months, respectively.
DST Begins DST Ends
Year at 2:00 am at 2:00 am
2000 April 2 October 29 = 210 days
2001 April 1 October 28 = 210 days
2002 April 7 October 27 = 203 days
2003 April 6 October 26 = 203 days
2004 April 4 October 31 = 210 days
2005 April 3 October 30 = 210 days
2006 April 2 October 29 = 210 days
2007 April 1 October 28 = 210 days
2008 April 6 October 26 = 203 days
203 days = 29 weeks
29 weeks / 52 weeks = 6.69 months
23 weeks / 52 weeks = 5.31 months