Hydrangea silk flower arrangements preserve very well, especially if you invest in very good flowers, you can expect them to last for years after their initial purchase.
To keep them looking good, make sure that you dust them regularly. Dust piles up on silk, weighs it down, and causes discolorations. Cleaning them a few times a week will help avoid this.
Secondly, keep your silk flowers out of the sun, which will cause them to fade. Lastly, avoid getting your silk flowers wet, which will cause them to get misshapen.