There are 2 more ways to set default alert time.
First way is that to set the default alert time in the iOS settings area, please check the path below.
iPhone settings>Mail, Contacts, Calendars>Default Alert Time.
Second way is being in the calendar website settings page, maybe you use Google or Yahoo or any other 3rd party calendars, if you use those calendars, try to visit the calendar website(i.e. http://calendar.google.com) and go to the settings page so then you can find the settings for alarms for each calendars. in case of Google, call it "Notifications"