How long is the email verification token valid for?

Hi, can anyone confirm how long the email verification token (email) is valid for, I can't seem to find an answer. We use Joomla 3.1.5 for our organisational extranet. User Activation is set to "Admin". Most staff have no problems in registering, verifying their email address and then my team activate their accounts (once their email addresses have been confirmed). However, some staff are saying that when they click on the verification link in the email they receive an error stating "verification code not found". One user registered on 1st July and only clicked her verification link yesterday (??) and received the error above. I suspect the verification code is valid for a set time period so I'm keen to find out what that is before I respond to her issue and manually activate her account. Any help would be appreciated.