When I have no other tests I do it as often as possible. From like 2PM to 11PM, with a lot of breaks of course (to listen to japanese music, to watch dorama, to eat, to take a walk, to dinner). It's not hard at all, at least for me. I really enjoy studying japanese.
I want to take 2kyuu this year, I'm going through soumatome 2kyuu bunpou and I'm still adding vocab from the 3kyuu, so I have a lot to study. I will start in 2 weeks to study tobira in a fast pace so that I can start "progressing from intermediate to advanced japanese). In a fast pace because I have finished AIAIJ (but in the first week I will go through the vocab and texts and listening comprehension, I only focused on grammar).
This study time includes anki (though most of anki hours are in the morning, around 1 hour), textbooks and listening to tv shows/dorama/music/podcasts (tokyo local, that I really, really enjoy) at the same time.
My problem with studying japanese is that when I have other tests I cannot focus because I'm so addicted to this japanese learning thing that I only want to study japanese.
I'm going to have a 5 week break from college and in those weeks I intend on finishing Tobira, at the very least. I also intend on adding the rest of the 3kyuu vocab and start adding 2kyuu. I will also start adding not 20 (like now) but 3 new cards a day or more, but this will increase my anki review time a lot so I don't know.
Long study hours are waiting for me
My motivation comes from my progress, I feel that my japanese is really improving thanks to these long study hours, learning grammar is getting easier and faster, I also really enjoy listening to japanese pop, watching japanese tv shows and doramas. I also have an awesome teacher.
Also, latety, the fact of having seen an add asking for someone who could speak japanese for a job, where I live (small town) has been motivating me. They offered 3250€ o.o (here in portugal an average salary is 700€, a good salary is 1000€ so...).
How to study for massive hours?
imo, you ought not to get bored. Whenever I get bored with studying from textbooks I just take a break or start singing songs while memorizing the lyrics or watch some tv show :p