Depends. The minimum to maintain fluency is highly individual, it depends on your current level. To maintain fluency, continue immersing as frequently as you can.
The longer you have studied Japanese, the longer you can maintain your ability without much immersion. If you've studied your target language for many years, you can even last multiple months.
The two easiest things you should to to maintain your ability are keeping up with your SRS reps and immersing passively. The SRS doesn't take up much time. In maintenance mode you're not adding any new cards so the number of due cards each day will be getting lower. Passive immersion is practically free, it doesn't take any extra time at all.
Then you should do at least some active immersion, and the exact amount of time is going to depend on how long you've been studying your TL prior to going into maintenance mode. I would say at least an hour a day is necessary.