What they will achieve...

I was looking some photos in flickr on yesterday, on description it said they used diluted developer and developing time running till 2 hours.

I was quite curious to know what kind of result they will achieve with such a long developing times.

How they determine the developing times and developer concentration for a given film speed?

Do a search here on Stand Developing - there's plenty of info as to the why's and wherefore's. The most common developer for this is Rodinal diluted 1+100 to 1+200.

Stand development should only be used to compress the tonal scale of a contrasty subject. It is not a general purpose developing method.

You can also use a 'water bath' process to achieve compaction. Here, you alternately place the negative in the developer for a short time, with agitation, then move it to a tray of water, where you let it sit. If you do this back and forth, because the highlights -- where there was more exposure -- will exhaust the developer more rapidly, you can build up proportionally more density in the shadows.

The risk is that you may get uneven development and/or what is known as 'bromide drag.'