i think it’s 18 hours straight. i had not actually seen this in the manual before. actually who am i kidding, i didn’t read the manual. but it does show that it recommends 18 on/6 off in order to maximize the lifespan of the fan.
i’ve been running it constantly but at level 4 for 18 and then level 2 for 6 hours. i suppose i’ll switch it to be off during those 6 now.