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Attenuation of cables is always a big concern, especially for telco companies which lay miles of them in order to reach a destination.
In your application, the attenuation will depend on a few factors which you will need to find out so you can calculate the total attenuation you will have. This will help you decide what path to take and how far you can go.
As a general rule of thumb, every cable has its own attenuation. This means that you need to find out the attenuation for the specific cable you are using.
In networks, where coax cable and serial cable is used, the attenuation should not exceed 50dB's, but this may not apply to your application and should be used as a figure to help guide you. Each connector you use, whether it be db9 or rj45, will add to the total attenuation the cable will have, so you should make sure you use as few connectors as possible. Again, the attenuation figure for these connectors is something you need to ask your supplier.
From experience I have seen extremely long serial cables work with one pair of computers, and fail when used with other computers, reason being that the signal that comes out of the serial port of each computer, is never the same. In most cases, the newer the pc, the weaker the signal coming out of the com port will be, and this should also be considered when making your cables.
On the other hand, there are devices in the market which can be used as signal reconstructors. These devices will help you overcome any signal losses you may have in specific points of the cable's length.
While I am unable to provide you with numbers and figures to help you more, I have pointed out a few important things you need to take in consideration. From there, you will need to do your homework and eventually will be able to calculate the attenuation your cable will have.
Good luck!
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