The consistency will vary depending on which brand of canned frosting you use. Some are thicker, some are thinner (but not runny). You'll need to experiment with your can before decorating to see how it behaves. If the frosting is too stiff at room temperature, simply pop it in the microwave for a few seconds to soften it up. If you have trouble piping with your frosting, you can also thin it out a bit by adding a teaspoon or two of heavy cream. Or, try spraying a mist of water from a spray bottle onto the surface of your cookies before decorating.