SUPEE-7405’s most important fix is that it blocks hackers from breaking into your admin and taking over your store. Clearly SUPEE-7405 is critical to have installed. Developers should estimate around 3-4 hours to install the patch locally, test and fix, push to dev, allow the client to test, and then deploy to production. However, below is a list of changes that may arise after SUPEE-7405 is applied. These will need to be evaluated on a site-by-site basis, depending on the extensions you have installed.