Each episode is 20 minutes long, and there are 90 episodes, so the series takes 20 minutes per episode * 90 episodes = <<20*90=1800>>1800 minutes to watch.
1800 minutes / 60 minutes per hour = <<1800/60=30>>30 hours to watch the series.
Tom can watch 2 hours of TV a day, so it will take him 30 hours / 2 hours per day = <<30/2=15>>15 days to watch the entire series.
#### 15