It varies widely from company to company. It depends on how dangerous of a bug it is as well as the exploitability; this determines the remediation period. Usually, very severe bugs like heartbleed or front facing deserialization are fixed within 1-7 days, whereas a CSRF in some random POST call could be 90/180 days depending on how a company treats less severe bugs.

You have a security guy assigned to keep track of bugs and is notified after the remediation window has passed. It's usually assigned to the person that found it because they are able to easily understand the issue and then verify the fix. They will confirm it has been fixed for all the machines it was found on, then close the ticket out. However, often bugs that are considered unlikely to be exploited will be kicked back, and long chains of emails between the security guys saying "Please fix this bug! I'm just trying to do my job", and developers saying "We have 0 dev cycles and a release coming up, not a high priority! Can't you just close it?"

This email thread goes on for weeks/months, and the security guys push the redemption period to help developers meet their deadline. It gets pushed back so many times that the security guys get sick of reporting it because they know it's a low severity issue, but still is a problem none the less. They push it up the chain of command and force the PM of the project sign off on the risk associated with what could happen if exploited, they almost always have it fixed immediately then it's closed.