Pen has 2 factors: skill and intelligence.

Failing the skill check means you can end up destroying the thing you're trying to write at any time. Once you hit your "max written characters" limit, defined by intelligence (can't remember if skill had any effect, didn't pay so close attention) the scroll gets erased when you write onto it and just says "The ink fades into nothing" which was annoying as hell when trying to be all artsy-fartsy and shit. This is probably what Rhyaldrin was referring to, the scrolls at least didn't fade by time back then.

Oh, and it *was* characters, not lines, that determines the limit.