When you can read Japanese comfortably. Not when you're still a total beginner because it will just make everything harder. But you want to do it sooner for additional Japanese gains. It will increase the time you spend with your target language.
User interfaces use a limited range of relatively specific vocabulary. If you want to speed things up, frontload the vocabulary before switching your devices. Here I have a list of sentences ripped from Android's system UI. Go through the list and use Rikaitan to make targeted sentence cards for words you don't know.