The default date range for iCal calendars is 0 days into the past and 3 months into the future, but you can set your iCal calendar date range to display events ranging from from 0 days to 18 months into the future, and from 0 days to 12 months in the past. To do this, open up the Settings tab in Calendar Manager, and find the "iCal Feed Range Settings" controls. Use the dropdowns to set how far in the past, and how far into the future, the calendar should look to find events.
Please allow about 15 minutes after updating this setting for the iCal feed files to refresh.
Note that the more events are on your iCal feed, the larger the .ical file will be; extremely busy calendars will not update end users as frequently.