Great question and there are a lot of factors that go into that but the short answer is basically  2 hours for every 50-75 guests.
Now that said… Let’s break down a typical wedding event with 100 guests (other types of events are basically the same)…
Our recommendation is to start the photo booth rental at the reception after the wedding ceremony itself. There is no point in paying for the photo booth rental only for it to sit idle while you are saying “I do”. You might have to pay for a little “idle time” if the wedding and reception is in the same room but it will still save you money.
Generally speaking, people go in the booth in groups of at the very least a couple. If all 100 of your guests enter as couples, then you’d need 50 sessions. Since our mirror booth does around 20 sessions per hour (3 minutes per session average) you’d need a minimum of 2.5 solid hours for every couple to get a print. If you anticipate your guests entering solo for their image or if they wish to enter multiple times, then you will need more time. Also if you plan to have the booth pause while toasts, cutting the cake, 1st dance, etc. are happening then you would need to account for this period of idle time as well.  Taking that into consideration we recommend at least 2 hours for every 50-75 guests attending so that everyone gets the opportunity to strike a pose.
Don’t be fooled by photo booth companies that claim they can do 40-50 sessions per hour. They probably can but your guests will feel like herded cattle being rushed through the process instead of having a fun and enjoyable photo booth experience.
So to summarize all that and to allow your guests to have the best photo booth experience…
50-100 guests  = 2-3 hours
101-250 guests = 4-5 hours
Never Miss a Moment
Of course, the best recommendation is to simply rent a photo booth for the entirety of your reception or event which ensures no moment is missed. Getting there early and staying late is an important feature the photo booth rental can provide because your guests will have an option all throughout the night to go and get their pose on! After all, it’s the one “take-away” from your special moment of a lifetime that your friends and family will treasure long after the cake has been digested.