Thankfully for Guy, Google Calendar's event reminders are even more customizable: now you can choose any period of time between 5 minutes and 4 weeks. This works not only for the default notifications settings, but also for individual events. As usually, you can choose to receive notifications by SMS, email and as Google Calendar pop-ups.
The default notifications are via email (10 minutes before each event) and pop-ups (5 minutes before each event), but you may need more time.
Tip: to change the notifications settings for a calendar, click on the small arrow next to the calendar's name and select "Notifications".
{ via Gmail Blog }