When does one become liable for wearing Sha'atnez? Is it merely putting the garment on one's body? Is it when the act of dressing is complete (ie., buttoning the last button, zipping up the zipper, etc.)? Or is there some measurement of time or distance walked while wearing the garment that effects the violation of the prohibition?
Shatnez is forbidden when it is worn as a normal garment -- i.e. to protect from the cold, rain and heat. It is therefore permitted to try on a new outfit for size, even though it may contain shatnez
“Shatnez is prohibited only when worn as an ordinary garment, for the protection or benefit of the body, or for its warmth, but not if carried on the back as a burden or as merchandise.”
12 Sifra, Deuteronomy 232
13 Talmud, Tractate Betzah 15a
It seems clear (as I have been taught too) that liability begins as soon as one gets benefit from the garment.