Seventh google link

http://veganyumyum.com/2008/10/homemade-vegetable-broth/
Cook for 1 hour, turning the heat down a bit once the whole thing starts boiling.

Sixth google link

http://www.simplebites.net/homemade-vegetable-stock-101/
Reduce temperature to medium and simmer stock for one hour.

Fifth google link

http://www.squidoo.com/vegetable-broth
Heat to boiling, then lower the heat and simmer for about an hour, stirring occasionally.

Fourth google link

http://frugalliving.about.com/od/makeyourowningredients/r/Vegetable_Broth.htm
Reduce heat and simmer for at least an hour.

Third google link

http://allrecipes.com/howto/making-vegetable-stock/
Allow the vegetables to simmer for an hour. Any longer than an hour and the vegetables will begin to turn mushy and begin to lose all their vibrant flavor.

Second google link

http://www.thekitchn.com/basic-techniques-how-to-make-h-136725
Cook for One Hour or So

First google link

http://www.simplyrecipes.com/recipes/how_to_make_vegetable_stock/

 Cook for 1 1/2 hours.