Is there a limit on the length of rg6 cable that you can run from a direct TV dish to dvr receiver?

100 feet maximum is your best bet. I'm using a 100 foot cable now with no problems. If memory serves me right though, 150 feet it about the maximum length you could use, but I wouldn't push it much beyond that without expecting some kinds of problems. Keep in mind that the quality of the cable you're using will vary from manufacturer to manufacturer and there are types of RG-6 that are of better quality then others.