How many hours should the open bar be?

Our bar was open till 15 min before the reception ended.

No one left at 9/9:30, even people we thought would leave.

Our reception is from 6-11 and we are having the open bar from 6-10. We figure people will start leaving around 10 and we are having a full open bar (this place is known for their strong signature cocktails), so ending the open bar around 10 seemed best for us.

Our bar was open for ~7 hours. Getting to our venue involved a ferry ride for most of our guests, and the best ferry for our ceremony time put guests at our venue at ~4p. So we opened our bar at 4p (and had some nibbles for them), which was a half-hour before our ceremony. We closed the bar 15 min before the end of the reception (11p). We have heard from several guests that they really appreciated it.

5 hour open bar here, we like booze

I think the typical/standard is 5 hours

We are doing cocktail hour prior to the ceremony (6pm) and keeping the bar (beer & wine) open until 10pm. bartenders are allowed to start serving at 4:45pm for early guests and are closing one hour from when the entire premises must be vacated. So total - 5 hours.

I'd say 5 hours is typical with a cocktail hour and 4 hour reception.

Shai, is your reception only two hours long?

We're having a 5 hour reception, first hour is open bar- wine, beer and liquor. Last four hours are open bar for wine and beer, with a cash bar for liquor. (Our family isn't huge on drinking, so beer and wine made the most sense, but we'll be picking up the tab for any liquor purchased, made more sense to potentially have a $40 bar tab vs dishing out hundreds for liquor that won't be consumed)

I'd suggest seeing if you could switch to a cash bar instead of closing it completely- but unexpectedly having to pay might also put a sour taste in someone's mouth... I'd personally have an open bar the whole time UNLESS you have a group of people that you know won't be upset by it