I'm quite glad there are others that are posting on this, and that are aware of this happening in our skies (domestic U.S.). I haven't got the exact timing down, but they always seem to spray in X shapes, and the dispersal of the chemtrails themselves can be anywhere around 30 minutes up to all-day.
In addition, I've noticed that the the jets that spray these chemtrails are at very high-altitude, and are hard to make out, even with high-powered binoculars. I have noticed this kind of spraying happening since the 90's, and always wondered what the agenda was behind it.
One of the more interesting videos on this subject is from a highly knowledgeable guy (Dex the Vulcan) who worked on Wall Street, and has been taken down repeatedly from youtube:
http://www.dailymotion.com/video/x4k30eu_the-zombie-apocalyps_news
If you have any pictures, accounts, and so on, please feel free to post and share. Hopefully we can piece this mystery together.