If kept in ideal conditions, dried flowers can last for many years - the colours may fade over time and they may shed if knocked. To ensure your dried flowers last as long as possible we recommend that you find them a home in a cold, dry room - ideally away from windowsills and extreme heat.
Make sure they don't get wet- if they get damp they will start to decompose.
If you're putting some outside (maybe on your door or as a table setting) bring them in overnight and in damp weather. Lastly, you'll also need to dust them fairly often, try using a light duster or a hairdryer. Following these steps, your arrangements should last 1-3 years, if not more!