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I know you can show focus limit in viewport by checking the Display > Limits on, but can you show the starting and ending points of focused area in viewport?
Now I have been rendering the image over and over again to check if I have long enough focus depth, and that takes too much time.
This Add-on allows to visualize the Depth of Field limits (near limit & far limit) in the viewport:
Once the Add-on is enabled, select the camera, open up the Properties region of any 3d View (N), scroll all the way down to the bottom and enable the visualization. From there you can fine tune all relevant properties of the selected camera:
Notice that you can disable the instructions to hit Esc in the Add-on Preferences if needed.
In addition to that, you can also set the focus distance by using the 3d Cursor:
Further reading: How do I set focus range?
I think you are confused with the terms:
Focus is just one point
There is no starting and end points, or long or short focus depth, or F-stop limits.
It seems to me you are asking about depth of field. Depth of field refers to the "circles of confusion" (as pointed out by @lbalazscs) The range acceptable focus depends on the focus distance, the length of the lens and the F-stop (or iris diameter) of the lens (and the magnification of the image in projection or display). Blender tries to follow the calculations for cameras in the real world. There are plenty of charts on the internet where you can get a reference for depth of field.
In blender there is no tool to show you a range of "acceptable focus", where the image is sufficiently sharp.
If you are using cycles however, you can enable depth of field preview for the 3Dviewport in the camera section's Depth of field and get a very good idea of how focus is working in real time.
Note that the viewport settings and the camera settings for rendering can be set separately.
Short answer: use the render preview, so that you don't have to render over and over again.
Longer answer: there is no such thing as "starting and ending points of focused area". From the Wikipedia:
Precise focus is possible at only one distance; at that distance, a point object will produce a point image. At any other distance, a point object is defocused, and will produce a blur spot shaped like the aperture, which for the purpose of analysis is usually assumed to be circular. When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as "acceptably sharp".
What we have instead, is the "acceptable circle of confusion", but what is "acceptable" depends on a lot of factors. The best you can do is to trust your eyes.
