Timing is of the essence.
If you are delayed for whatever reason you will miss the show.
For Mount Bromo this should be your rough schedule:
3:00 AM - Leave Cemoro Lawang for the Mount Bromo Sunrise
5:00 AM - Arrive at the topmost viewing point
5:30 AM - Watch the Mount Bromo Sunrise
6:10 AM - Begin your decent
7:45 AM - Arrive at the base
8:00 AM - Freshen up and head to Bromo Crater
9:00 AM - Start the Mount Bromo hike to Bromo Crater
10:00 AM - Arrive at Bromo Crater
10.30 AM- Begin your decent
11.45 AM - Back in Cemoro Lawang for breakfast
A lot of people make the mistake to stop in Cemoro Lawang for breakfast after the sunrise hike. I highly recommend to carry a snack for the road and continue straight to the Bromo Crater, if you don't need a break. The later you go, the hotter and harder it will get and therefore more difficult to climb up.
For Kawah Ijen this should be your rough schedule:
10.30 pm- Begin your car journey from Bondowoso
12.30 am- Reach Kawah Ijen base
1.30 am- Begin your hike
3.30 am- Reach the rim of the crater
3.30 am- Continue the descent down to the crater
4.30 am- Reach the volcanic crater
6.00 am- Begin your ascend to the rim of the crater
7.00 am- Continue your descent down to the base
8.30 am - Reach back to the base
No matter what happens you must reach the crater before dawn or else you will miss seeing the blue sulphuric flames.
If you must dodge your way through the crowd, don't hesitate to do so. I had to do it and it was totally worth it. A lot of people snoozed and missed the entire show!