There's no functionality to stop restricting the access after 7 or 14 days. This is because a public user doesn't have an account or a subscription so there's no subscription date to even check against.

Is it possible to wish this feature for next updates?
- to realise a limited in time restriction for content?

Yes, I've already added a feature ticket in my last reply for this. You can find the feature ticket below.

forge.joomlapolis.com/issues/7924

- a content restriction with conditions f.e. a special content custom field

You can already restrict content based off the value of a field using IF substitutions shown in the substitution tutorial below. See the bottom for examples.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb

Basically you're needing the below usecase as I understand it.

"If the article date is less than 14 days old then check if I am a subscriber. If I am a subscriber then display the article text. If I am not a subscriber then send to plans purchase page. If the article date is older than 14 days then simply display the article."

There's just not a way of doing that at this time. Mainly due to IF substitutions not being able to do datetime comparisons unless specifically coded to do so (e.g. like CBSubs IF) and CBSubs IF conditions being restricted to the subscription date. Will carefully consider what needs to be implemented for this usecase to work in a future release. It maybe possible to just implement it in CBSubs Content itself so it's a simple parameter selection so substitutions don't even need to be used.

www.joomlapolis.com/documentation/283-cb-paid-subscriptions/tutorials/18240-time-released-subscription-content-solution