Your Outlook Exchange Calendar options can be configured to allow colleagues to view a minimum of 12 months of future availability.
By setting your Free/Busy option to 12 months, you will not affect the permissions granted to individuals as recommended in the previous section, rather you are making a global setting which allows 12 months of your calendar to be available for viewing. It is important to note that, once this setting is applied, a colleague will not see your availability for a meeting more than 12 months in advance but will, however, be able to invite you to a meeting beyond that time period.
From the Tools pulldown menu, select Options.... then click on Calendar Options, then click on Free/Busy Options. Make sure that 12 months of Calendar free/busy information is available on the server.