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.
There's no easy answer to this question, as the amount of time it takes to determine the status of graylisted emails can vary greatly depending on a number of factors.
Graylisted emails can take a while to process, and it's often hard to tell whether they've succeeded. If you're waiting on an important email from a graylisted address, you may want to check back after a few hours or even a day to see if it's gone through.
It's generally advisable to give yourself at least 24 hours after sending out an email campaign before checking the status of any graylisted emails. This will give the system enough time to properly flag and filter any messages caught in the graylist, and you should be able to get a more accurate picture of which emails were truly delivered and which weren't.
If you are a bulk email marketer or using a paid email list, you must verify your email list using bulk email verification tools. Some tools like ZeroBounce identify greylisted emails and provides accurate results. You can also check the great list of bulk email verification tools to identify greylisted email addresses.