After purchasing a product you can download upgrades that contain bug fixes at any time within the subscription period (typically 12 months). If you purchased a developer plan you are entitled to lifetime upgrades.
Please note that the upgrade is defined as a revised release of a product, with bug fixes and/or new features as they become available, for the same platform (e.g. the same Joomla! version) that the product has been advertised and sold for.