I've got the seidio 3500mah battery. The only times it's died where when I tried and tried to kill it. Running GPS status, wireless teather, etc. However, the other day, I unplugged it at 100% charged at 6AM, I watched a movie on my sd card that was an hour and a half long, three 20 minute episodes of south park on the sd card, surfed the net and streamed certain videos for about an hour. After 6 hours, I was at 70%. Everything was updating regularly, 2 hour intervals at most. Kept mobile networks always on. No wireless or bluetooth. Love the extended battery. I can actually go places with this phone now!
In the end, it really depends on how much you use your phone. However, it will blow your factory battery the **** away! Just charge it fully, use it normally, let it die and charge it again a couple of times.