Dried flowers can last for a long time even years. On average around 1-2 years they are at their best but to get them to last much longer follow these steps:
Avoid direct sunlight. Dried flowers do naturally fade over time some more than others and direct sunlight speeds up that process so keep them in a shaded area to help maintain their colours.
Handle with care and as little as possible to help prevent breakages.
Do not water and avoid humidity. Keep dry to prevent mold or mildrew
Minimal dusting as dried flowers are fragile. If you want to dust them you can gently blow on them or you can use a hair dryer set to cold on the lowest setting then carefully use from a distance to gently remove any dust off the flowers. Another way is with a soft paint brush gently dust the flowers though do this with caution and as carefully as possible.