As long as you have the Enterprise version, it will not expire and you will contiune receiving the definitions updates to protect your network.
But it's recommended to renew as soon as you can to keep this licensed & valid and to receive support from Symantec in case you need them.
However if it's a new install then it's 60 days, Small Business, 30 days and upgrade from previous version - 90 days.
This is based on https://support.symantec.com/en_US/article.TECH230601.html under "Managing Endpoint Protection licenses" (almost bottom of the article)
Hope this answers your question.