Question: > 
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> [Is there a 'rule of thumb' that I can use to estimate depth of field while shooting?](https://photo.stackexchange.com/questions/6047/is-there-a-rule-of-thumb-that-i-can-use-to-estimate-depth-of-field-while-shooti) 
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How do you estimate the aperture needed to produce sufficient depth of field for a given subject?

For example:  

Suppose I am facing a bicycle from the front and use whatever combination of focal length and subject distance is necessary to have the bicycle completely fill the frame. I focus on the handle bars as I would like this to be the sharpest point of focus. How can I best estimate the aperture needed to ensure that entire length of the bicycle is within acceptable focus?

I have tried using my camera's DOF preview button, but the resulting image is too dark in the viewfinder to determine the depth of field.

I could use a DOF calculator, but this would take too long.

I could take a few shots at varying apertures, but again, this would take too long.

What methods do you use to visualise depth of field in situations like this?

Answer: 
Experience! Once you've taken enough photos at various apertures and distances, you will have the experience to get the DOF right on the first or second try, without having to measure distances or perform calculations. With an accurate enough estimate, bracketing should get you quickly to the right aperture. Many cameras offer auto-bracketing options, so if you're able to do a relatively accurate guesstimate, then you can fire off three or five bracketed shots with one press of the shutter button.

Until you have the experience, you're either going to have to make guesses and review the results in the viewfinder (my recommendation), or get out the tape measure and use a calculator if you really require precision.
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> What methods do you use to visualise depth of field?
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When I used a Nikon FE, I used to look through the viewfinder and press the "stop-down"/"depth of field preview (DOF)" button. 

None of my cameras since have had one. It is certainly a feature I miss. Nowadays I mostly guess.