Someone recently asked me about this in the comments section of a post. I am posting an edited version of my response here for easy reference.
If you play myVEGAS a lot, it is easy to rack up a lot of LP’s (Loyalty Points). That lets you buy a lot of Rewards, but MGM Resorts/Mlife limits you to using no more than 3 MGM Resorts Rewards in a 30 day period. That 30 day period is based upon when you use the Reward. It is not based upon when you use your LP’s to get the Reward or the day that you call to make a show or room reservation with the Reward. They consider you to be using the Reward on the day of the show, the day you check in to the hotel, the day you go to the buffet or restaurant, etc.
You can use your LP’s to purchase more than 3 Rewards in 30 days. You just can’t use more than 3 in a 30 day period.
Let’s say you purchased Rewards for a room at the Mirage, tickets to LOVE and a pass for the Mirage buffet. You check into the Mirage on November 1st and go to the show and buffet on November 2nd. The 30 day period for 1 Reward starts on November 1st, when you checked in. The 30 day period for the 2 other Rewards starts on the 2nd. You would not be able to use another Mlife Reward until December 1st. On December 1st you would be able to use 1 Reward. On December 2nd, the additional Rewards would be available.
You can still purchase more Rewards during that period and you can still make reservations using those Rewards. You would not be able to actually use them during that time, though.
Carnival Cruises has its own 3 in 90 limitation which does not count towards the MGM 3 in 30 (and visa versa).
The Monorail, Wolfgang Puck, House of Blues, Uniquely Vegas/Aussie/Cali, Couples Resorts, Departure Lounge, Gray Line Tours, Smith Center and Hexx Rewards available on myVEGAS do not count towards the 3 in 30 days limitation.