Currently Google Now notifies me 10 minutes before I need to leave the house for an appointment (taking traffic into consideration). How do I change the notification time to some other value?
The default reminder time is actually based on your calendar. If you open your calendar, open Settings, and select General settings, "Default reminder time" is near the bottom (I'm working from Cyanogenmod 10, but AOSP/Nexus devices should be similar). This controls the general notification time. The Now functionality is separate, and (from what I understand) is based entirely on travel time. If your appointment has a destination that Google recognizes, it will calculate travel time and pop up a card far enough ahead to allow you to meet that travel time and arrive on time. I'm not positive if it factors in your reminder time into this or not, but it's possible that it is. If that's the case, it will notify you 10 minutes + travel time currently, and if you modify the calendar notification time, it should modify the Now notification time. Again, this is speculation based on how I understand Now to function, but I believe it's fairly accurate based on my experience.
The default reminder time is pulled from the Calendar's settings, if you tap the event to open it in the calendar editing screen after it's been recognized by Google. However, if you just tap "Create Event" after it's been recognized without reviewing it (as mentioned above), it will default to 15 minutes. This is on a Razr Maxx HD, Verizon, ICS.