I think two hours are about right...
An hour may not be enough to greet everyone, wait for the 'politely late' folks, get the meal and cake and any gifts all done...
However: I will mention this..
I disagree that people will naturally leave when appropriate.
That has not been my experience.
And, there was even a piece about this on one of the Dear Abby type columns.
I wouldn't go on that assumption.
When my son was young, we had a very nice party for him.
Several cars lined up in the driveway.
At one point I asked one person (probaby family?) if they might move their car so others could get out if/when they were ready to leave.
I believe on person thought that I was 'rushing guests away'.
But, IMHO, this kind of 'party' is not like an all day casual get-together affair.
In fact, I was at a party on Saturday where this was a bit of an issue.
So, I might just go post an 'Inspired By...' thread.