One of the main issues that amateur players deal with is being able to maintain their top arm speed throughout the entire baseball season. Hopefully, most players have been in a resting period with light throwing for the past the couple of months.  This is not just for pitchers but for position players also.  You should find out when your school tryouts begin and when the first week of games will start.  Most players will want to be in throwing shape at the start of the tryout period.  If you back up 6 weeks from when you want to be ready, you will have a start date for your throwing program.  If you are a pitcher you may want to start 8 weeks before and take in to consideration the weather in your area.  If you are a position player only, you can leave 4-6 weeks for your throwing program and be ready to go.