Licences for using OLS expire when you have not activated them within the time frame set by your sending institution or organisation. This means that, after a certain date, you will no longer be able to access OLS in order to take your language assessment or language course. This date is set by your sending institution or organisation and is indicated in the invitation email that you have received.
Language courses are accessible for the duration of your mobility, but you need to activate your licence within one month of receipt of the invitation. Therefore, if you go on a mobility period with a duration of six months, you need to access the language course within one month of receiving the invitation. You will then have access to the OLS language course for the same duration as the mobility period (e.g. six months), starting from the activation of your licence, thus for six months from your first access to the language course.