Lare normally warranty for one year. The amount of time you can expect should be in the range of 1500 to 2000 hours.
I don't know of any replacement lamps that are built to last longer.
I can try and help with the lamp part number and best place to order from, but I will need a better model number. The one you gave me did not show up on the MItsusbishi
Please recheck model number.
The replacement lamp can be purchased through a number of Sources.
Also google replacement lamps for model WD 60735
this way you can compare prices at different parts distributors.
Most offer a one year free replacement warranty.
If you decide to replace the lamp yourself, be sure to follow instructions
that come with the replacement lamp and do not touch with bare hands.
Oil from you hands can shorten the lamp life.
I found that there a number a complaints to Mitsusbishi on this and similiar models
with the lamp going out.
You might want to check this out for yourself.
It will cost you less for the bare bulb.
If you are handy at small projects, it is fairly easy to change just the bulb.
If you buy the complete housing its just one screw open the cover on the back
pull the housing and bulb out and plug the new one in.
Be sure to replace cover securely befo trying to turn set back on.