I used to do a lot of video transcoding on an older AthlonXP system. It regularly ran at 100% for many hours at a time. This went on on a fairly regular basis for months or maybe even years and the CPU was still going strong when it was replaced. I built an HTPC a couple of years ago that records probably 10 hours a week from the antenna. It is setup to downsample and compress everything it records. It has been doing this for 2 years now and is still going strong. I think you're fine. If it does shorten the life of your CPU at all, it's unlikely its shorter life will be any less than the time you will use it. I think you're fine.
Saying that though, it might not be a bad idea to get Speedfan or something like it to keep an eye on your CPU temps and fan speeds. If nothing else you can reassure yourself that it's not getting too hot.