The limit on subscription payments is 1 year. 

The reason for this is that some payment methods (such as PayPal) do not support longer subscription periods, perhaps because terms over one year would have a high rate of failures due to credit card expirations. If products could be configured for multi-year subscriptions and someone switched to or added a payment method that does not support multi-year subscriptions, then members who attempted to sign up using those payment methods would receive an error.