You may be new to qi gong practice and you may be trying to figure out how long you should practice for. It is quite easy to answer- everyday!
While some exercises, such as running or working out, are best to do every other day, qi gong should be practiced daily for best results. It is often difficult to fit a long class into a daily practice which is why I like to focus on short routines. Experience has shown that you get more benefit from short routines more often than longer routines practiced less frequently.