Standard game times
Weekday night games: 6:00 PM first pitch (most common). Weekend/holiday day games: 2:00 PM first pitch. Weekend night games: 6:00 PM (same as weekday). Some Saturday games start at 2:00 PM or 5:00 PM depending on the team and TV schedule.
Gate opening times
Stadium gates typically open 2 hours before first pitch. For a 6:00 PM game, gates open at 4:00 PM. For a 2:00 PM game, gates open at 12:00 PM. This is important because arriving early gives you time to explore food stalls, buy merchandise, watch batting practice, and find your seats.
When do games end?
Average NPB game length is about 3 hours 10 minutes. So a 6:00 PM game typically ends around 9:00-9:15 PM. A 2:00 PM game ends around 5:00-5:15 PM. Extra-inning games (up to 12 innings max) can add 30-60 minutes. Games tied after 12 innings end as a draw.
Planning your evening
For a 6:00 PM game ending at ~9:15 PM: you have about 2-3 hours before last trains (typically 11:30 PM-12:30 AM). Plenty of time for a post-game meal. For restaurants near the stadium, make a reservation for 9:30 PM to be safe.
Day game advantages
Day games (2:00 PM) are perfect for tourists: they end by 5:30 PM, leaving your entire evening free for dinner and nightlife. They're also less likely to conflict with other plans.
Checking schedules
NPB schedules are posted on npb.jp (Japanese only — use Chrome translate). Each team's website also has their full schedule. Games are rarely rescheduled except for typhoons at outdoor stadiums.