I want to force users to set up some expiration date for every survey. I am able to make a plugin that makes use of beforeSurveyActivate-hook and can get the ID of the survey by $this->getEvent()->get('surveyId'). But how to get the expiration date from that?

Given that surveyId, you should fetch the survey model and get the expiration date attribute from it.

Something like this:
>>>
$S = Survey::model()->findByPk($iSurveyID);
$expirationDate = $S->expires
>>>

Thanks, this works.

Somewhat hard to find instructions for developing plugins. LimeSurvey itself is great!