I’ve been reading some more research on extensive reading and readability lately. One paper showed gains in reading rate, vocabulary and comprehension with students reading about 150K words over 15 weeks at an intermediate level. This was contrasted with another study where learners read ~65K words over 28 weeks and failed to show improvement. I think there is probably a threshold of some kind where you need to read a certain amount per week to improve language skill. The amount probably varies with the level of skill you already have. Someone still improving their knowledge of the most frequent 400 words of the language will not need to read as much to achieve vocabulary gain (assuming appropriate graded readers) as someone reading at the 2000 word level. The study that showed gains had students reading with vocabularies of 800+.

Given the 10K words per week guide, and the typical reading rate in foreign languages often being around 150 words per minute, that equates to about an hour of reading per week, or 10 minutes a day. That’s not a bad aim for maintaining and hopefully improving your language skills.