That depends on the dojo. The customs after bowing in and out of the dojo vary greatly. Some dojo bow to every black belt entering or exiting the floor, others don't. To them it's a waste of time bowing that much rather than training. You'd never stop bowing and would pay more attention to black belts entering and exiting than your training.
Some dojo have a Shinza and you bow to that.
I bow in and out of the dojo, to my Sensei when I greet him or after my Sensei gives me personal instruction. That's it in my dojo.
When other Black belts enter
Thats if you know. The reasons.
Your masters and uke or your friend that you gonna use the technique on him/her.
Okay, so I know that I have to bow entering and leaving the dojo. When else do I have to bow?
