When I first posted messages to comp.lang.postscript about my efforts to write a postscript interpreter I was informed that it takes approximately 5-10 man-years to write a fully-compliant level-3 interpreter. And I misread it as 3-5 years and happily set about my task. And after 5 years, thinking I should be about done, I re-read that old message and became enlightened (as to the true importance (and correctness) of that estimated quantity of work).
In like vein, were I to set about re-implementing the basic TeX engine (in my fancy favorite language, of course) how many years is it gonna take?
Presumably, the time should be rather low, since the source is published as a literate program. But my pascal is a little rusty.