My peace Lilly (of only five months) leaves are turning brown at the end and are limp. This plant has sentimental value and I want to keep it alive.
How much and how often should I water this plant?

Peace lily needs water and the soil should be kept moist. The ideal temperature is 60 F. This plant likes bright indirect sunlight. The leaves will turn brown at the tips if it has too much light, insufficient water or too much fertilizer. Humidity can be accomplished by placing the pot over a tray with water and pebbles so the pot does not touch the water.

I have found that a peace lily is very hard to kill. Unless it is not watered properly or it is placed in full sun.

When Peace Lily leaves go limp it is because they are not getting enough water. That could because you are allowing the soil to get too dry between waterings. Or, it could be because the soil has been kept constantly wet and the roots have gradually suffocated (rotted) and are no longer able to absorb water for the plant.

Water yours thoroughly. If it perks up, then you know you have allowed it to get to dry and will need to water it more often. If the leaves don't perk up after watering, then it is because the roots are in bad shape and no longer functioning properly. Without a photo, it is hard to know if recovery is likely.

In general, Peace Lilies should be watered just as or before they start to wilt. Add enough water so that some water trickles through the drain holes. There is no precise formula for watering as each plant is different and should be watered as needed.