What's longest i/c's can be run?

Please advise me what is the longest practical length that interconnects can be? Is there a performance difference, because of length only, between rca's and xlr's? Your comments and suggestions are appreciated.
I run about 20 feet with no problems. I have used both single ended RCA and balanced. The balanced have a little more gain, but other than that not much difference. I believe you are always better to run longer interconnects and keep your speaker cables short as possible. There would be a greater deterioration in sound with long speaker cables than long interconnects.
I fully agree with the above noted comment. Keep the speaker runs as short as possible and your interconnects can be as long as you want. My experience is quality cable is an important factor in letting your equipment reach it's potential.
As long as you use a low capacitance cable you should have no problem with long single ended runs. If you go balanced it will make no difference at all what the length is and it is generally thought that with balanced cables the quality of the cable is not as important as with a single ended cable. Some balanced circuits will have more gain, but that is only an issue if you are not using a preamp (direct out from CD). There are some tube preamps (Counterpoint comes to mind) where you may experience some roll off of the higher frequencies with long cable runs, so you might want to check with the manufacturer, but I've never had a problem with this and I've had several tube preamps.
Ibid. The best set-up is long interconnects and short speaker cables. Balanced is best, because you do not have as much chance of loosing gain, or gaining noise/interference. Generally, single-ended should be no longer than 8-9 meters. Video cables, especially the infamous s-vhs cable, will suffer noticeably with longer lengths. Component video is much more immune to degradation with length. My component video is about 45 feet, and still looks very good. If you use balanced cables, Canare is very good quality, and they do not pay extraordinary sales commissions or advertising, which makes up much of the cost of the "audiophile cables." There is a person on Audiogon that advertises good quality, custom cables--charles sprague. I have been very happy with his cables, and I have very expensive gear that would show any sub-par interconnects.
If you must run long runs of interconnects and you have the option, balanced is better. Balanced interconnects are used in professional circles for just that reason. More gain, and less likely to act like antennas for the unwanted garbage floating around. Balanced connections are not a panacea,how ever. A well executed single ended system can easily out perform a poorly executed balanced one. It is much more expensive to build a truly balanced system than a single ended one. Some times the money may be better spent elsewhere. Good listening.
I use 40 foot single ended runs of Purist Dominus RLS interconnect between my preamp and mono amps. However, I do agree with the above comments about capacitance with tube gear, and with the cables needing to be the best quality for this long run. I am using a tube preamp and mono tube amps, and have not experienced any problems. Note though, that the Dominus has very low capacitance, less than 12pf per foot. This makes the 40 foot run less than 475 pf total! Not enough to cause the high frequency roll off that could occur with some cables. You can find the capacitance of your chosen cables from a specification sheet, or call the manufacturer for the numbers. The input impedance of your amps plays a role too. The higher the input impedance of your amps, the longer the run can be before it becomes a problem. Most are 47K ohms, 100K ohms is easier to drive for a long run. These numbers are not relevant for balanced, as a true balanced load should be 600 ohms. Atmasphere for instance, is a truly balanced load, and is 600 ohms on the balanced jacks, but is 100K ohms on the single ended RCA inputs. An excellent example of design to avoid this type of problem. (PS, I do not use Atmasphere, just a comment.)