If you use Wilton's class buttercream, it stays good for a long time and you can use it over and over again to practice with, no refrigeration needed because there is nothing in it to spoil.
Stiff consistency Class buttercream
1 cup shortening, 1 teaspoon vanilla, 2 Tbsp water. Cream together, then add 4 cups powdered sugar, 1 Tbsp meringue powder and a pinch of salt. Blend until creamy, only a minute or two. Makes 3 cups icing.
To make medium consistency add 1 tsp water to each cup of stiff consistency icing. To make thin consistency, add two teaspoons water to each cup of stiff consistency. I always add just a small amount of piping gel to my thin consistency. It is easier to pipe leaves and practice writing with the piping gel added. I just keep it in a tupperware container and label which consistency it is, then when I want to practice, I can.