Do you have a page reference for this? I'd like to read up on it.
I'll do my best.
Believe it or not, but this is 4E's rule on powers:
"Unless otherwise noted, a power is instantaneous and has no lasting effect."
--PHB, "Durations" section, page 278.
Hardly satisfying, is it? Saying Wild Shape lasts throughout the encounter (or until you use it again, of course) seems not only to be reasonable, but it also seems to be the intent of the designers (to have the power last, that is).
However, if you're looking for a rule saying it doesn't last all day, you won't find it. I'd go so far as to say you would make an unreasonable demand if you were to say "unless I see a rule telling me Wild Shape is limited to encounters I'll rule it works all day".
That is because 4E isn't about things outside of combat; it's
all about the encounter. (Yes, this can be a disappointment for some D&D players.)
The best I can do is directing you towards the following, and noting Wild Shape isn't (a ritual):
"Rituals (see Chapter 10) can create effects that last for hours, days, or years."
--PHB, "Durations" section, page 278.
Really, that is all there is to it. If you choose to remain unconvinced, I'm afraid there isn't any more pages to turn to.
Instead I'd like you to do a little exercise:
you show
me where it says Wild Shape
does work outside of combat!
(Because that's really where the burden of proof should lie, considering the 4E design parameters)
Anyway, it's your game and you should feel free to do whatever if fun for your group. What you shouldn't do, however, is complain about how overpowered out-of-combat Druid Wild Shaping is if you choose to allow it...
Best Regards,
Zapp