HDMI cable length...how long is too long ?

Here you go. Special wall plates exist so that you can "home run" cables without needing to terminate at the plate and then add more cables again. This prevents signal loss with a home-run connection. For example, to run HDMI Baluns with Cat6 you cannot cut the connection with a wall plate. So installers use one of three options:

1) An open plate. You can chose different diameters depending on the number of cables. An example is here: http://www.monoprice.com/products/product.asp?c_id=105&cp_id=10425&cs_id=1042509&p_id=3997&seq=1&format=2

2) A plate with a bristle-style opening. See the second plate in this picture: http://www.lowes.com/pd_12862-60784-HT2001-WH-V1_0__?Ntt=legrand+hdmi&UserSearch=legrand+hdmi&productId=1214179&rpp=48

3) A plate with a rubber cover with slits in it. http://www.monoprice.com/products/product.asp?c_id=105&cp_id=10425&cs_id=1042505&p_id=4653&seq=1&format=2

So using this kind of solution is really ideal with HDMI installations. Additionally, it allows you to install a pull string and put a conduit behind it for future wire run pulls with ease. As you can see, there's a very easy and simple way to do this and also make it look aesthetically perfect.

I like those wall plates that were in the links , thank you .

I'm still undecided whether to use the suggested HDMI cables or to use cat 5/6 cables instead . Are there any futureproof advantages of one over the other ? That would probably be the deciding factor .

Thank you .

I strongly suggest running both. However, for your purposes, I'd simply use the HDMI cables for now and then if you run into problems look to use a BALUN on the Cat6 cabling. Rule of thumb: You always want a home run to eliminate points of failure. The Balun is great, but unless you need to use it, I'd stick with the HDMI runs with quality cables

Ok , thank you Internetmin . I appreciate the info and advice .