It varies based off the lamp model, lamp manufacturer, the ballast starting method, and as Rommie mentioned the switching frequency. Here's a list of some common lamp life ratings, all of which shuld be current production lamps. All are currently produced Philips 4100K lamps.

T12

F20T12/CW ALTO
@ 3 hours per start - 9000h

F40T12/CW Supreme
@ 3 hours per start - 20000h
@ 12 hours per start - 24000h

F96T12/CW Supreme/ALTO
@ 3 hours per start - 12000h

F96T12/CW/HO-O ALTO
@ 3 hours per start - 12000h

T8

F17T8/TL841/ALTO
@ 3 hours per start (prog. start) - 30000h
@ 12 hours per start (prog. start) - 36000h
@ 3 hours per start (ins. start) - 24000h
@ 12 hours per start (ins. start) - 30000h

F32T8/TL841 PLUS ALTO HV
@ 3 hours per start (prog. start) - 38000h
@ 12 hours per start (prog. start) - 44000h
@ 3 hours per start (ins. start) - 30000h
@ 12 hours per start (ins. start) - 36000h

F96T8/TL841 PLUS ALTO
@ 3 hours per start - 24000h
@ 12 hours per start - 30000h

F96T8/TL841/HO/PLUS ALTO
@ 3 hours per start - 24000h

Modern lamps, actually are rated to last quite a long time.

No idea where the other messages in this topic went but I think it's also important to mention that there are two common ways used when manufacturers calculate lifetimes. One is for 10% failures and the other one is for 50% failures, meaning how big percentage of lamps will reach that value. Needless to say 50% survival rate isn't that realistic in many real world situations.

There is also the "installation lumen drop to XX%" criteria, standardized for LEDs (there the standards saet the limit to 70%), but used for already a long time for installations using scheduled group relamping servicing style (but the percentage threshold varies).

For the "50% survival" criteria: This criteria is useful where spot relamping (replace immediately just the lamp that has failed) is used, it tells how many lamps you will be replacing.

i tend to pick up all the free tubes i can off craigslist (businesses tend to put them up after doing led retrofits) and i've encountered some that really have put in their time and yet still keep kicking. in particular i've been very impressed with philips f34t12 energy saver cool white tubes.... the phosphor gets all dingy and yellow-brown, the ends get dark, they have cathode spatter all over the interior of the tube (the full length, not just the ends) and yet i've installed them in frequently-used locations and they just go and go and go. once they illuminate they look just fine although with the lens off the fixture you can see all the cathode spatter still. i just relamped 12 2-tube fixtures with them (all used in the condition described) and they start right up and look great after a minute or so of warmup time. the colour rendition is mediocre and i would not choose them for use behind gels (i prefer f40t12 cw supreme for that application) but they are great for general illumination. when i first acquired those tubes i thought they were junk from the physical appearance but there's clearly some good engineering that went into them.