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

internetmin said it best.

Run MANY cables

Each HDMI cable should be one long cable not going through intermediate wall plates. Two or more.

Run 2 or more Cat6E with wall panels

Run run through wires that allow you to pull new wires through the walls should the need arise.

If you are going to use wall plates, I recommend using those which convert HDMI to Cat5/6E.

HDMI cables aren't the most robust for traversing distances and IMHO having more male/female adaptor connectors in between is likely to cause even more things to go wrong.

A friend of mine experimented by soldering HDMI cables directly to the AVR and projector (some old sets) and found that by removing the connectors and soldering directly, he was able to get a long HDMI cable run to work where using the connectors failed to establish a sync.

I have used Wireworld 5 series up to 12m and NuForce up to 15m and they work up for HDMI 1.4 1080p24FP video.

But even with those cables, I don't recommend using HDMI cable from AVR to wall panel -> HDMI to second wall panel -> HDMI cable to projector