When more information comes to light, and we figure out exactly what is causing the bricks, it will prove that the length of time used has no bearing on whether or not you are likely to brick. And, who knows? It must just be the length of time. We simply don't have enough information yet to pin it down.
The only common factors we have found so far is the firmware. My 3DS hasn't bricked, and I have spent roughly 50 hours on it since I got my Gateway card. Played at least 60 different games on it, on two different microSD cards, and even swapped out the 3DS SD card, repeatedly.
I've written the contents of the Blue Card with the Gateway files over at least a dozen times so far, and I have probably cold booted my 3DS and launched Gateway Mode at least 200 times.
Why, then, have I not bricked?