There is no defined time limit for a 302, rather than a clear header response for search engines that the redirect is temporary not permanent (which is a 301 header response). By default at some point you should correct to a 200 response or a 301 response. You could in theory leave the 302 but it is not good practice.
Having said that my thought would be how trusting a visitor would be being sent to a different domain mid way through browsing.