Depends on the **** to work ratio. I predict you will get about 2 months.
I guarantee you will want a new one before you actually need a new one
Depends greatly on how you use it. If it's used in bed or on your lap etc then it'll heat up more while in use and cause damage faster. If you move it around a lot the charger cable could break easier. If you use it on battery mode and semi drain the battery a lot then it could cause the battery to die sooner (better to completely drain the battery then completely charge it rather than half way). If you download loads of programs and heavily use the processor and memory then it'll wear down faster, whereas if it's more just word processing and internet browsing it'll last longer.
If you use it conservatively, on a desk, always using the charger cable, then you should expect a good 5 years from it easily, quite possibly even significantly longer, though I don't know a figure for the average. Probably better to get a new one after 3/4 years anyway as the software and technology will have moved on significantly by then.
any second now
16 months before you start running in to major problems with your laptop/computer
i've been using mine for six years now , aside from the loud noise of the fan and the always dead battery , it runs great
Given my heavy use/abuse, I usually get three years out of one.