Standard (or "category 1") HDMI cables have been tested to perform at speeds of 75Mhz or up to 2.25Gbps, which is the equivalent of a 720p/1080i signal.
High Speed (or "category 2") HDMI cables have been tested to perform at speeds of 340Mhz or up to 10.2Gbps, which is the highest bandwidth currently available over an HDMI cable and can successfully handle 1080p signals including those at increased color depths and/or increased refresh rates from the Source. High-Speed cables are also able to accommodate higher resolution displays, such as WQXGA cinema monitors (resolution of 2560 x 1600).The HDMI specification specifies required performance of a given cable, but does not govern the length as long as the cable meets the given requirements. So how far of an HDMI cable can you run from your video source to your television set? Monoprice technical support recommends 25 feet as a practical limit for their high speed cables to carry both a 1080p and 3D signal.
After 25ft they are no longer certified for high speed. Standard speed cables can carry 1080p but it depends on the device is why we only list them as 1080i/720p. Many newer devices though if all you want is 1080p can achieve that with standard speed cables, you only really need a standard speed cable if you are running both 3D and 1080p. - Nick M., Monoprice technical support
Think of 25 feet as the magic barrier. You could go longer than that but you'd need to confirm that all of your devices will work with standard speed HDMI cables in order to view 1080p content without concerns.
So what to do if you need to go longer distances and insist on carrying a 1080p signal? All hope is not lost. You have a couple of options, including more HDMI cables and cat6 ethernet cables. Remember that HDMI certification is based on performance, so if a cable is capable of carrying that full signal then it still passes certification, no matter the distance. Monoprice carries ultra high performance HDMI cables with "RedMere Technology" that has a built-in chip inside the heads of the HDMI cables that cut down the thicker high speed cables and still allows for full 3D and 1080p. These cables are mono-directional and use the chips to allow for the full resolution over the longer distance. You can also use CAT6 ethernet cables which allow for runs up to a whopping 330 feet. You'll need two ethernet cables connected to a wall plate or extender.
CAT cables are rated for about 330 feet, give or take - depends on your setup, how you have them ran, STP or UTP, solid or stranded, etc. And also the extender you use. - Nick M., Monoprice
So you're covered for most practical distances. Just make sure that your cable matches your application and that all specs are up to speed!