Projector lamp brands will provide an estimated lifespan (in hours) based on the manufacturers testing. Whilst sometimes we can get unlucky and have a lamp fail earlier, there are some things we can do to help extend our projector lamps' life as much as possible.
Most projectors have a feature where they enter a cool down mode after you turn them off. In this mode the projectors fans will keep running, cooling the lamp evenly, while the lamp is off. To make sure you don't damage or even shatter the lamp you should keep the projector plugged in until the projector completely turns off and fans stop.
While the projector lamp is still on (even while it is powering down), the lamp is heated to hundreds of degrees, in this state the lamp is very sensitive to shock. Moving the projector while the projector lamp is still on (or when it is very hot) could shorten the lamp life, or worse, it could cause the projector lamp to suddenly break.
Most projectors can operate in temperatures ranging from 0°c - 40°c. They can usually be stored in temperatures above 40°c but it is not recommended. If a projector has been in extreme cold (below 0°c) you should let it adjust to normal room temperature before turning it on. The lamps can also be sensitive to cold air (from air conditioning vents) being pumped directly onto intakes.
Pay attention to where the exhaust fan is on your projector. It is best to ensure no objects are near the exhaust that could restrict hot air escaping. Most brandsa recommend at least 30cm of space around the projector to allow for hot air to dissipate. Projectors will overheat if hot air cannot disperse.
The projectors will suck air in for cooling purposes and unfortuantely can also suck in dust. Some models use filters to keep the inside of the projector clean while others use a sealed optical block where a filter is not required. If your projector has a filter it is recommended to check it a couple times a year and give it a clean if necessary. If your projector does not have a filters you should clean (with a damp rag) the intakes and exhaust ensuring no clumps of dust get sucked into the unit. A small amount of dust visible inside the projector is not of huge concern though, never open the projector to clean... this can cause significant damage.
Many digital projectors offer what is called "Economy Mode" or "Eco-Mode". This is a setting that reduces power to the projector lamp, which in turn reduces the brightness of the image. Using the projector on an Economy Mode can extend your lamp life, usually by more than 20%. The great thing about the Economy mode is you can easily change back and forth between Economy or Bright modes whenever you like, so you can use Economy at night and perhaps change over to the Bright mode for daytime use.