You don't have to follow the Duolingo suggestions for strengthening your skills. You can just move on if you yourself feel confident about your skills.

If in the course of the subsequent lessons you find that you have difficulties with certain topics, you can just choose these topics on your own in order to strengthen them in a more targeted manner.

Duolingo just assumes that you will forget topics over time, and so after a certain time, it will suggest (by reducing the golden circles) that you should retrain those skills. Another hint for Duolingo are the mistakes you make and the vocabulary hints you take by hovering over the words. These are indicators that you may not be sufficiently familiar with the topic, and Duolingo will suggest refreshing this topic quite soon.

But if you think that you still remember the skill, you can move on.

In addition to the good answer of Heike333145:

... criteria Duolingo uses to determine when a lesson needs to be refreshed.

http://duolingo.wikia.com/wiki/Decay

My favourite method:
https://www.duolingo.com/comment/6176795/Best-Way-to-Make-Your-Tree-Turn-Gold-and-Stay-Gold