There are two kinds of invitation and they behave differently.

The shared link — the one you copy from the invitation dialog — does not expire. It works until you rotate it or switch it off.

• Generating a new link kills the old one. There is one link per teacher account, and refreshing it replaces the address. Anyone still holding the previous link gets Invalid invitation link, with no explanation of why — so send the new one to anybody you had already given the old one to.
• Switching the link off is reversible. Turning it off stops it working; turning it back on brings back the same address, not a new one.

An emailed invitation expires after 7 days. After that the student has to be invited again. An invitation that has already been accepted cannot be used a second time by somebody else.

A student who opens a dead link of either kind sees only Invalid invitation link. The page cannot tell them which of the reasons applies, so it is worth saying in your message that a link has been replaced.