Return time, the one estimate a booking depends on
Your return time is the minute you expect to be inside your own room with the door closed, and every booking made on the way back is planned from it.
Of all the facts in a first message, the return time is the one that shapes the booking. It is not the time the evening ends, or the time you intend to leave. It is the moment you expect to be standing in your room with the door shut behind you. The rest of the plan is counted backwards from there.
How to estimate it honestly
Start with the leaving minute: the time you will actually walk out of the building, after the coats, the goodbyes and settling up at the table. Add the journey, measured at the hour you will make it, which after midnight usually means walking or waiting for a night bus. Then add ten minutes for the room. That total is your return time plus the buffer, and it is almost always later than the first figure that comes to mind.
Why it goes in the first message
A message that says only where you are staying and how long you want leaves the reply guessing whether you mean now or in an hour. Writing the return time with minutes in it, 23:20 rather than about eleven, lets the booking be timed properly the first time. If you are unsure, give the earliest and latest you think likely.
When it changes
It will change. Dinners overrun and friends order another round. Write as soon as you suspect the new figure, while she has not yet set out, and the hour can usually move with it. The same-night change entry explains what that costs at each stage.