I know it can be a bit of a pain, but to make your flowers last as long as possible, do the following:
When you get home, recut all the stems at an angle, with a sharp knife (or scissors in a pinch).
Remove any leaves that will be below the water line, as they make the water dirty and prevent the stems from sucking up more water.
Place flowers into cool, fresh water in a clean vase (it should be clean enough that you would drink out of it).
For the longest “vase life”, keep flowers in a cool area of your home, out of direct sunlight.
Change the water every 2-3 days, and recut the stems.
If your flowers came with flower food/powder, follow the package instructions- this really is food for your flowers.
Depending on the variety, most fresh, local flowers should last a week or more. There are exceptions, like poppies, sweetpeas and a few others.