We currently only register and renew domains automatically for one year at a time.
We've found that longer registration periods lead to a higher chance of customers losing or forgetting their account details or missing notifications and ultimately letting their domains expire due to outdated contact information or expired credit card details.
The annual notifications serve as a reminder of sorts to keep everything up to date. Or, if something unexpected happens and the domain is no longer needed, it can be cancelled with no time/money lost.