A lily stays generally 10 to 14 days in the vase. For a good flowering result, there are a number of tips to keep them fresh as long as possible.
Lilies like fresh water, best is to refresh the water every 3-4 days, otherwise the stems can become clogged. Also make sure that the water is not too dirty so make sure that no leaves are hanging in the water, otherwise they will rot and create poor water quality. There are special tablets you can put in the water, this chlorine tablets keep the water clean and pure so you do not need to refresh that often.
Best is to cut occasionally the stem for about a centimeter, so the stems do not become clogged and can lead the water through more easily. When you do this, cut the stem as the pictures below, this promotes the taking in of water because the contact surface is larger.
It is also best not to place the lilies in the sun behind the window, they will fade more. Take a semi sunny spot in the house.