With today's malts a 45-60 minute malt is perfectly adequate (20-30 minutes in many cases) - but a longer mash won't hurt and may increase you efficiency if you can keep the temp steady. If the temp drop lower for an extended period you could get a drier beer than intended. I don't think that enzymes get denatured very quickly at mash temps.
FWIW I mash my tripel for 90-120 minutes and get very high efficiency. Or, at least a few points higher than normal.