Here's how you can change the default alert times on your iPad, iPhone or iPod.
1. Go to Settings.
2. Go to Mail, Contacts, Calendars.
3. Scroll down to the Calendars section.
4. Tap on Default Alert Times.
5. Tap on your preferred default alert time.
For more, see the original article at the link below.
Setting Default Alert Times in iOS | iLounge