How long does a smartphone battery last?

It's 10 am and after receiving a long phone call (15 minutes) I'm just with 46% of battery left. At 10 am. Just three hours after disconnecting it from the charger, I'm with less than a full charge.

On the left there is the battery stats from Battery Eta. If you don't count Sunday ("Domenica" - I went to the beach and I left the phone charging the whole day), the average duration is disappointing... 12 hours. It means that if during lunch break I forget to charge my phone, after dinner the phone is dead.

It's the battery? No, it always behaved like that. Tthis phone (T-Mobile myTouch 3G Slide), when is not in a good reception area (see the screenshot, I only have one bar), increases the transmission power to a ludicrous level, eating battery like hot cakes.

In optimal conditions it lasts about 48 hours, but I would have to move in a new house, so it's not a good solution.

I could disable the Gmail push mail, but at that point I could carry a cheap dumbphone with infinite battery.

How long does the battery last to you? I'm curious to know if I'm just an isolated case or it's normal that nowadays a phone have to last just half day.