No really good answer for this. GIGO is the rule. Look at it much the same as DVD ripping and shrinking. If you start with a high-bitrate high-quality image, there’s a lot more room for shrinking. Compare to DVD, most of which vary around the 8Mb/sec rate with variable bitrates from 3-10Mb/sec. You will be displaying your image on the same size monitor as you do with your DVDs. The larger the monitor, the higher the bitrate you will prefer.
You can also consider creating 720p, if your tools allow that, and really shrink the files a lot.
Will you notice a difference between 15Mb/sec and 25? Depends on your monitor and your distance from it. I would think that 15Mb/sec at 1080 is a good starting point, but your choices are limited by the tools you use for encoding. The material itself is a big factor. 2 hours of talking heads needs a lot less bits than complex or high-motion video.
All of these numbers should probably be reduced when using MPEG4 codecs.