How effective are whitening toothpastes work at whitening teeth?

In fact they are not effective.
They will remove the superficial spots.
To really white a teeth it is necessary a professional work.
Whitening toothpaste aren't even able to remove all types of spots, for example nicotine.

They basically scrape the tooth surface, so the discoloration will go away.
If the discoloration is deep inside the tooth they are of no benefit.
The professional whitening done at the dental clinic removes the staining from the tooth, not the tooth it self.
With repeated use, obviously, they will remove most of the "white" part of the tooth exposing the "yellow" part.
So over time your teeth will get yellowish and yellowish.
These whitening toothpaste generally are not recommended.

Whitening tooth pastes are generally considered not very effective to actually whiten the teeth. They may be beneficial to remove some external stains but if the teeth are internally discoloured, they can only be improved by a prefessional who can prescribe home whitening or the whitening done in a dental surgery. 

No such whitening toothpastes are their ,its only a marketing skill to market their product.Only a professional can work out in the clinic for whitening of teeth.

Limited effectiveness up to 60% if used regularly and respect conditions 

whitening toothpaste wont enhance to color though they just prevent from forming newer stains.plus people seem to brush more often if they have a whitening paste than if they use a regular one. 

it never works .
If you are a smoker it will remove the nicotine color only

Whitening toothpates are abrasive pastes which "scrape" off the outer layer of stained enamel. If used wisely and not too frequently (according to instructions provided by the manufacturer), they are effective and remove the stains quite adequately. Only when the user applied excessive amounts, or uses the paste more times than recomended or applied excessive pressure while brushing will they begin to have detrimental effects of the teeth resulting in erosion of the outer enamel layer which may even result in exposure of the dentin layer which may lead to dental sensitivity. In addition, excessive use may lead to the demineralization and softening of the enamel layer ultimately resulting in carious lesions (most commonly class 5 lesions in the facial ascpects of the anterior teeth.

well , u have first to distinguish between whitening and removing discolorations, whitening is a process by which u change the color of your natural teeth to another and brighter value and shade and this has to be done professionally (most processes takes up to1 hour in office) or can be made using home bleaching (whitening kits Commercially available and the active constituent is CARPAMIDE PEROXIDE and it takes up to two weeks of usage before visible effect can be noticed) , but the whitening dentifrices available in the market only remove coffee and nicotine stains and not to full extent and by all means do not change the color of your teeth, and beware of those containing hard particles that would depride your superfacial layer and takes longer time to have visible effects mainly those containing baking soda as as an ingredient they are safe to use.