How long does MAP sensor for LPG last in years or miles?Mine failed after only 19.300 miles (31.000 kms),but it is clean inside.Is it normal?

Sensors usually die a prematurely short life from higher under hood temperatures. Idling cars excessively doesn't blow the hot air under the car since it isn't moving forward and the heat just lingers under the hood and affects the life of sensors and seals and bearings and hoses and so on.

Perhaps, cleaning the gas pressure regulator could remedy the cause of the failure. The rubber gas line can de-vulcanize from being left under pressure and accumulate oil in it, contaminating the pressure regulator. Turn off the gas, undo the gas line from the regulator and replace it or clean it with brake cleaner spray. Clean the MAP sensor. Maybe it will remain clean after all that.

You will likely die when you have to replace the engine

Don't worry about it, maybe there was something wrong with that particular sensor.  You can buy one for cheap on ebay.

it appears 30k km's what a car how worried are you going to be once its done 70 or 80 thousand k's?

NO number is accurate.

Even if you ignore the variability in lifespan of the sensor, it CANNOT be measured in years OR miles. Engine run time in hours is the only semi-valid measure, and it isn't consistent enough to track.

No, it's not normal. They should last many times longer than that. 

19300 miles.  Your car is fixed dude.  Drive your car.  Do not waste your time on something that is not important.  You will likely die when you have to replace the engine.