I think that a good metric to go by would be to actually list out the Display time and Voice Calls time. Barring the presence of Maps or another battery munching program showing up on the Battery Use list, these two seem to be the big culprits.
For me, I'll either get about 3 hours of Display time (with almost no Voice Calls), or about 2 hours of Display time with around 1 hour of Voice Calls. The numbers seem fairly consistent, with my combined use time (between Voice and Display) being around 3 hours.
With Social Networking set to update only over Wifi, no Live Wallpaper, Auto Sync off (but Background Data on), I can get any length of usage time I want, provided that I don't greatly exceed this 3 hour combo.
As an example, here's my current battery stats:
1 day 5 hours 2 minutes since unplugged
Voice Calls 57% (1 hour, 36 mins)
Display 25% (2 hours, 7 mins)
Cell Standby 17% (1 day, 22 hours, 22 mins)
(Time without a signal = 39%)
Phone Idle 4% (1 day, 2 hours, 54 minutes)
Battery is currently at 30%.