Your hop garland will hold its colour better if it is positioned out of direct sunlight and in a position where it won’t get touched or caught in a draft, all of which will cause petals from the cones to drop.
How long a garland lasts is also a matter of preference. Traditionally, hops garlands should be replaced each year at the hop harvest. Some people leave their garlands in place for several years and enjoy their golden colour. Most people replace every year as there is nothing nicer than the fresh scent of new hops and the opportunity to have a little autumn clean.