From the LMA, North America website:
LMA-Classic, LMA-Flexible, LMA-Unique, & LMA-ProSeal
The maximum duration for which an LMAairway can safely be used is not yet known; however, there is increasing evidence that the LMA airway may be safe for elective procedures in healthy patients lasting 4 to 8 hours in the hands of experienced users. If the LMA airway is used for prolonged periods, the respiratory function must be closely monitored, and a heat and moisture exchanger should be used. Also, intracuff pressures should be checked periodically and maintained at 60 cm H2O. Nitrous oxide diffusion tends to cause a rise in intracuff pressure in the LMA cuffs made of silicone. This may result in malposition, postoperative sore throat, or other adverse events.
Brain AIJ, Denman W, Goudsouzian N.G., LMA-Classic and LMA-Flexible Instruction Manual, LMA North America, Inc.
Ferson D, Brimacombe JR, Brain AIJ. International Anesthesiology Clinics: The Laryngeal Mask Airway. Lippincott-Raven, Volume 36, Number 2, 1998.
I always considered it safe and defensible for extended periods, probably for an upper limit around 3 or (cringe) 4 hours. Have used them semi-regularly in 2 hour cases with wide ranging success (N = approx. 300) Funny deal about cuff pressures though. Never seen a manometer in any OR....just the occasional ICU, and even then, RT usually had them hidden away. Just anecdotal evidence FWIW.