If you are an owner of the Dart account or have admin priviledges, you can see your plan and billing details.
Find your plan and billing information from your Settings (at the bottom of your menus to the left of your screen):
Your current subscription level will be highlighted, and as an admin with privileges, you can adjust your subscription level.
Billing can be set to monthly or yearly payment cycles. If you need a business or enterprise-level subscription to Dart, contact sales.