What defeat? Sakura is greate ninja who learn a high-rank teqniuqe , most of k11 doesn't even stand a chance against her but let's not forget that she needs alof time to prepare it, and if anyone was defeated it's only you while you couldn't even prove your point haaa
But I did prove it through Tsunade genius. I saw it happen right there in the manga.
Took her years the first time she says, and she has used it up a few times in the manga and regained it pretty quickly.
The process is different once you have it because Tsunade getting it back right away does not mean it's full but it does aid in proving the follow up.
Before Sakura activated the seal the chakra was in her forehead but it wasn't the required amount of chakra for the seal to appear and the seal itself acts as a container which basically helps the user in using the chakra for things such as remote healing and byakugou.
Now if you understand that so far, Now that the seal is already activated even the smallest amount of chakra the starts to build back up, the seal will appear. Between pain arc and the war Tsunade accumulated enough chakra to fight in that was against madara in the war for an extended period of time.
The same woman who said it took her years initially. There's no reason by these manga based events to believe it takes the same amount of time that it does the first time