I don't believe there are written rules to address this.
However, I would consider both actions you describe as roughly equivalent to retrieving an item from a pouch, and therefore say that it costs a minor action.
As for how long it takes in "real-time", the most accurate statement anyone can make is that it would take no more than six seconds - which is the approximate time given for one round of combat. With the exceptions of rests, rituals, and travel across distances, most actions do not have any rules stating how long they take to complete.
This does beg the question though, "Why do you need to do this?". As far as I'm aware, there's no effective benefit or penalty for using one hand or the other for most purposes.
Unequipping your shield is a standard action, according to the Rules Compendium's section on Shields (p267):
To use a shield, a creature must strap it to the forearm. Doing so is a standard action unless otherwise noted. Removing a shield is also a standard action.
Unfortunately, this means you won't be able to use a standard action for an attack in the same turn unless you spend an action point.
Once your shield is removed, your options are these:
- Drop your shield (free action), equip your weapon (minor action). If you want to attack, spend an action point for another standard action. This, however, risks someone else stealing your shield if you leave it unattended.
- Stow your shield (standard action), and equip your weapon (minor action). That's a second standard action! You'll need to bust out action points to make this quick, or prepare yourself to use this method over the course of two turns.
I'm not aware of any feats to speed up this swap!
Getting the shield back after dropping it
If you do #1, you'll need to recover your shield at some point. Picking something up is a minor action - this does not equip it. Once it's in your hand, it will take a standard action to either stow it away on your back (or somewhere out of the way) or equip it. Being a shield, it seems you can't simply stow it away as easily as other items, which normally take a minor action.
Where is this all from!?
This is sourced from various excerpts from the Rules Compendium's explanation of a Standard Action, particularly these sections:
- Equip or Stow a Shield (standard action)
- Draw or Sheathe a Weapon (minor action)
- Drop a Held Item (free action)
- Pick Up an Item (minor action)
- Retrieve or Stow an Item (minor action)
You should pick up that book, since it contains the most recent version of all of 4e's rules, complete with all errata and updates released since.
Best Answer
In general, it is a standard action to remove your shield from your arm. From the Rules Compendium pg 267:
If you choose to remove and stow it on your backpack, that is described specifically on pg 245 (Equip or Stow a Shield ) as also being a standard action.
It appears the hard part is unstrapping it from your arm. Putting it someplace once it's off is the easy part.