Like January, February is a good time for visiting Japan as the weather is usually sunny and dry and sightseeing spots are not crowded. The downside of a visit in winter are the relatively short days (sunset is around 5:30pm in Tokyo) and the vegetation's barren state.
1 A healthy person drinking approximately 2 liters of water per day makes 800 to 2000 ml of urine per 24 hours (4).  If the urinary system is healthy, the bladder can hold up to 16 ounces (2 cups) of urine comfortably for 2 to 5 hours (5).
Chemical enema or just warm water, they work pretty much immediately. You're just flushing out your insides. With a water enema, depending how much you use and therefore how high up it goes, it might take 20 minutes or so for you to expel all the water. You should be OK in much less than 2 hours.