This can vary depending on the greylisting settings configured on the target mail server.
You should use a professional email verification solution such as Email Checker, who have advanced greylisting techniques within their processes in order to get accurate results from mail servers that adopt greylisting.
You can sign up for a free account here: login.emailchecker.com
Yes, you might get a different result if you re validate after a while. Also, it is not that if you validate now an email and it is showing a status, and you re validate after a month for example, and you receive another status, that does not mean the first result was not correct. At the time, it was, but the status of an email can change the next day or the next year. That is why it is best to validate your list a couple of times a year, to see the evolution of your list. For this you can use an email validation platform like ZeroBounce.