Insert the following parameters into your iCal link to specify days ahead and days back.
&ical_days_ahead=300 (will allow you to pull specified # of day's events in advance)
&ical_days_back=100 (will allow you to pull specified # of day's events in the past)
Here is an example link including these parameters:
Note: be sure to insert your unique calendar identifier name where is says "yourcalendaridentifier"
More info on customizing iCal feeds:
Create an iCal feed for a single category or combination of categories:
Create an iCal feed for private categories: