I've got a 170 that only works in about 5 kts or less wind. It stretches back to the helm. My 150 works in about 7 or less and the 120 is good for up to about 15. I switch from a Jib to a storm jib above 18-20 or when my limited expierience sailing makes my spidey sense tingle with impending doom or too much heel.
Most of the time I use a 120.
My dad tells me switch to new sails and/or reef the main when:
Uncomfortable in the cabin
Lack of steering / hard to control
He also showed me that nothing smooths a boat down like taking 2 knots off of the speed and 10 degrees from the heel!