This is working well if all of your layers use the same spatial reference. If some layers are not in the same spatial reference as the map you will have to project the layer extents and to union them by code.
If you are talking about the full extent of a layer, you can look at this legend sample, the 'ZoomToFullExtent' method is wired up the Layer.Initialized event and zoom to the full extent of that layer once the map spatial reference is known. A geometry service may be used to project the layer full extent to the map spatial reference.
Hit the Full Extent button in the Tools Toolbar. It looks like a little globe.
If that is taking you out to far you can right click on your most extensive feature and click zoom to layer.
You can manually adjust it to where you want to be. Right click on the Data Frame header. Defaults to Layers. Open the Data Frame Tab and click on the Other and Specify Extent button. Then choose Current Visible Extent.
Save your MXD. Then every time you click on the Full Extent Button it will take you to the same spot.
I want to zoom to the full extent of all the layers that I have currently loaded.
This is working well if all of your layers use the same spatial reference. If some layers are not in the same spatial reference as the map you will have to project the layer extents and to union them by code.