That depends. Always cut off the faded flowers but you can wait to shorten the long stem because with some orchid varieties if it is not the winter dormant season you may get a second flush of blooms. Just wait to see if the stem stays green and plump.  If you notice the stem drying up then trim it back to within one inch from the base or stalk of the plant. Looks like you have the perfect spot with bright, indirect light for your orchids. How lovely. Keep growing, Marianne Binetti