How long does a mushroom trip last?

Your experience may vary: 

It is a general consensus that a Cubensis (the most popular magic mushroom)  trip usually takes about 45-60 minutes to come on, and lasts around 6 hours if the dry mushrooms are consumed by chewing and swallowing full mushrooms. This is a general timeframe and each person may notice effects for a longer or shorter duration. Far more subtle effects may still be observed following the normal 6 hour timeframe.

The length of a trip depends primarily upon the method of ingestion, and not so much upon the amount ingested. Regardless of the method of digestion though, psilocybes are pretty consistent as far as how long the effects last. When mushrooms are powdered or infused into a tea the onset is much faster, the peak hits harder and with more intensity, and the overall length of the trip may be reduced.