It depends.
At work we cut mostly 3/4 plywood and partical board on a large commercial machine. You'll probably get a bit less life on a small, slow homebuilt machine.
To make it last as long as possible
Don't plunge straight down into the work, always ramp in.
Cut as fast as possible with the lowest possible spindle speed. Heat is VERY bad.
When you're cutting at high speeds, you can get better tool life out of larger parts, because if you have to start and stop alot like you would on small parts, you never really get to cut at the high speeds you'd like to, so tool life will suffer as well.
With a brand new bit, I probably cut 30 to 40 4x8 sheets worth of parts. This is usually at 300+ ipm and 16,000 rpm. With a 10HP router, I can use the bit a little longer because I have the power to push it through the work even when it starts getting pretty dull.
I also cut Maple and Oak up to the full depth of cut of the bit (~1-3/4"), in multiple passes. I usually try to use a new (or just resharpened) bit for hardwoods.
I usually get these resharpened 2 or 3 times, too. But it's not cheap (~$40, I think), and the more you sharpen them, the shorter the life.
I've also used them in handheld routers (with guide bushings) with great results. You wouldn't believe how much easier and faster they cut than a standard 2 flute straight bit.
And one last thing. Vortex gives large discounts on multiple bits. Sometimes on only 2 or 3 of the 1/2" bits, and you save about 30%. (kind of like buy 2 get one free). On 1/4" bits, the price break is around 7 bits. Just increase the number in your cart until you see the price break.
Oh yeah. If you think you can just get any carbide spiral bit and get the same results, you won't. I've used standard 2 flute spirals from Bosch and others, and compared to the Vortex bits, it's like night and day. If I need spiral bits, I'll never buy anything else. But I haven't tried Onsrud, and I hear they're quite good as well (and they have a HUGE selection
http://www.onsrud.com )
Hope this helps.