If I stick to the built-in timer (the toothbrush switches off automatically after 2 minutes), I can brush my teeth at least 25 times.
With the included case, I can charge the toothbrush on a USB charger when I am out and about. This works well with a mobile phone charger, as the mobile phone also needs to be charged regularly (usually daily).
From this point of view, I find the built-in battery more than sufficient. I would buy the toothbrush again at any time.