class My_Custom_Block extends Mage_Catalog_Block_Product_View_Abstract {

    protected function _construct()
    {
        // the $groupId make it so each customer group will be cached separately
        $groupId = Mage::getSingleton('customer/session')->getCustomerGroupId();
        $this->addData(array(
            'cache_lifetime' => 5600,
            'cache_tags' => array(Mage_Core_Model_Store::CACHE_TAG, Mage_Cms_Model_Block::CACHE_TAG),
            // you can add anything to the cache_key to make it cache separate versions for different customers and stores
            'cache_key' => "custom_unique_name_{$groupId}"
        ));
    }
}