Hey Laurens3103,
Thank you for using the Apple Support Communities. It sounds like you're charging your MagSafe Battery Pack for your iPhone, and you'd like to clarify when you can determine that it is fully charged. We can help!
This article has a lot of helpful information related to your question: How to use your MagSafe Battery Pack. As quoted from within there, "While your MagSafe Battery Pack is charging, the status light might flash. If it flashes green, your battery pack is fully charged. If it flashes amber, your battery pack might need more time to charge." We encourage you to also read the other sections in that article.
Let us know if you have any further concerns about this.
Best regards.