I'm sorry, but you don't seem to have understood the question.
The question is not about "time" restriction, but "Credit" restriction. Credit means (usually), you have to pay for it, or Buy, credits.
You then assign a simple amount of needed credit points to a course, event, seminar, etc. and people can use their credits to pay for, and enroll.
It's dead simple and common sense.
Myself I am looking for days now. But moodle appears to be set up for school like institutions, only, in a form of "intra-learning". In this case, the "student" would have paid already for attending that school/institution, hence, courses are included.
In that way, it's not a true enterprise solution for a global market. Even schools organise "outside school" activities, events, workshops, etc., to which a student would need to apply and pay extra.
But moodle doesn't seem to have ever thought about that.
Am I wrong? Please educate me. I'd be more than happy to learn that I am wrong because that would only mean I can utilise moodle.