Certain fonts can in principle be recognized with next to 100% accuracy, though even this depends heavily on the source and capture.
In real life, the Bavarian State Library did a project with 20th-century monographs (good print quality, simple layout, common characteristics, modern spelling) and the outcome was > 98% percent word accuracy, which enables numerous retrieval scenarios as can be seen in the search interface (German only). This was obtained with images in greyscale made by a scanrobot and with commercial OCR software - results can be much worse with free OCR software, but this depends highly on factors such as language support and level of post-processing etc. The more tailoring and post-processing involved, the better the result.
Most importantly, as already pointed out, for use in retrieval, (non-stopword) word accuracy is what you should mainly be interested in, and which is naturally lower than character accuracy - and not even all OCR software provides this metric.
It may also be noteworthy that OCR software generally just reports "suspicious" characters or words, which in fact can be right or wrong, thus only giving an estimate of the actual number of errors. In order to measure that, comparison against ground-truth (the 100% correct text) is the only way - the impact competence centre has plenty of information on this, as about OCR in general.