Before we get into this...if you are in Vegas with kids, do not go to the Mob Museum. The material is definitely not suitable (how could it be!?) and I do not think they allow strollers.
We purchased the combo ticket for Mob Museum and Neon Museum, of the two the Mob Museum is a far more extensive collection and at minimum you will need two hours, but allow three so that you don't feel rushed. Visitors with a strong interest in mob history could probably spend 4 or 5 hours. The 41,000 square-foot building includes approximately 17,000 square feet of exhibition space on three floors. The Museum is housed within the historic former federal courthouse and U.S. Post Office. The courtroom on the second floor is the very courtroom where one of 14 national Kefauver Hearings to expose organized crime was held back in 1950.
It is a fascinating, very well executed (no pun intended) exhibit. The final exhibit is a sound booth where you can record a personal experience related to the mob/criminals. Selected videos are available for viewing, they are interesting and sometimes funny. As always, the tour ends at a gift shop where you can purchase books and the usual tchotchkes.
If you go for the combo ticket you do not have to visit both museums on the same day, also for the Neon Museum best to pre-book. All Neon tours are docent-led and often are booked in advance. Mob Museum is self-paced but can get crowded.