It's going to last 5 minutes or until the end of the encounter.
If you do it outside of an encounter, it will expire before your next encounter begins.
This completely answers my question. You can't simply have a permanent cart of battle standard. I forgot about the (until end of encounter means 5 minutes) clause. Thanks
As far as placing a standard on a cart while in combat goes, I have seen some interesting interpretations here. I would allow it. Although the term 'ground' appears in the description of the item (a poor choice of words IMO) the general description of standards says the can be "set into any solid surface that is not difficult terrain". That's pretty unambiguous.
The section that says you "plant the standard in your space or an adjacent square" but then goes on to say "until the battle standard is removed from the ground".
If the person writing this section hadn't been drunk, they probably would have written "when the battle standard is removed from this space". The term 'ground' isn't even defined in this instance, and in any case the standard was never placed in the 'ground', so it can't be removed from the 'ground'.
I think the person that wrote this entry is the same one responsible for the 'expertise' feat.
So then the answer is that you may place a standard on a wagon, but if the wagon moves such that the standard is removed from that space, the standard is no longer activated.
Also, you could say that you can place a standard in a space before a combat, and it will continue to take effect for up to five minutes if the combat starts soon (unless a rule I missed forbids you using 'rest of the encounter' powers outside of combat). It even makes sense flavor wise.
"Stand fast soldier, when the orcs get through that door they will not take this ground!"
Finally, if the wagon were the centerpiece of the battle, I would allow the zone to move with the wagon (but not extend outside of the wagon). When a battle takes place on a moving object I treat the object as stationary and the terrain moves by on the battle board. A group of players who hop aboard an enemy wagon going full speed down a bumpy road and manage to plunk down a banner on top of it deserve the bonus. This doesn't fly RAW but I like it.