I Have seriously read about 1/3 of SICP. Seing this level of dedication and actual time required is very reassuring. I found many of the exercises to be be deep, deep rabbit holes where you eventually had to set a time limit for moving on if you have any ambition actually completing the book. The depth seems to increase for every chapter! You could probably spend a lot more time than 729 hours and it would still be both a productive use of time and well worth the investment knowledge-wise.