Welcome to the violin @stephie, I hope you're really enjoying it! It's been a while since you started this post, so I'll assume that everything is coming along nicely and you're really enjoying yourself, or you've smashed your violin into a thousand small pieces.

As a physically active guy, I'll echo what everyone else said in that you should take it slow, and allow your body to build the required strength, flexibility, and nervous system function to play for longer periods of time.

That being said, everyone likes to measure their practice in "time." It's a simple, objective measure we can use to try to show progress. I read a great article a while ago however, and I think EVERY violinist should read it. It's here: http://www.bulletproofmusician.....-practice/

To summarize the article, "time" is not NEARLY as important as "focus." What's important when practicing is that you're truly focused on what you are trying to do and you have a specific skill you are working to improve when you are practicing. "Mindless practice," as it is called in the article, is about as valuable as not practicing at all.

So, what I'm trying to say, is don't worry about time. Worry about building up your body to play the violin, take time to focus during practice and avoid distractions and mind wandering, and just enjoy yourself. The rest will come.

Disclaimer: I'm a complete novice.