The spirit of the 50% rule is to influence the way we think about how we engage with art and creativity, to balance out the constant need to improve and to be doing something "worthwhile" (in other words, not wasting our time) with the opposite. It's not about timing yourself, or anything specific - it's about ensuring that first we rehabilitate those students who've developed extremely unhealthy relationships with learning through traditional academics, and those who see drawing as only valuable if other people value the result.

I'd say you can "stop doing the 50% rule" once it has fundamentally wormed its way into the way in which you engage with your creative pursuits. If you find yourself drawing for the hell of it regularly, without resistance, but rather actively wanting to do it, and if you find yourself not being frustrated with things coming out poorly, then the 50% rule has done its job. At that point, you're in a good position to decide how you divide up your time for yourself.

I would however continue to hold to it intentionally as long as you're working through this course at the very least, because those habits, and the damage the expectations and factory-production-line strategies traditional academics impose, are not easily undone.

I'm not quite sure what you mean by "does the 50% only counts for when drawing at home".

As a side note, I noticed that you appear to be posting each individual page of your Lesson 1 homework as a separate submission, resulting in a ton of partial submissions. This isn't really how the website is intended to be used - you're much better off just waiting until you're done a Lesson in its entirety, submit all the pages to a single imgur album, then use the link to the imgur album to submit a complete homework submission as explained in this video from Lesson 0 (at the 13:56 timestamp).