Complex sentence: However, at the same time the size of the code for general purpose operating systems was growing rapidly, and by the time UNIX-based code had reached 500,000 lines of code its maintenance and use was a challenge.
Split into simpler sentences: However, at the same time the size of the code for general purpose operating systems was growing rapidly. By the time Unix-based code had reached 500,000 lines long, its maintenance and use was a challenge.