The question is how much you want to risk.
If you bot 18 hours a day, 30 days a month, that's 540 hours a month botting. 24 hours a day, 30 days, would be 720. If you use a generic figure (say $1/hour), there's an obvious benefit seen to 24/7.
The problem lies in how much of a target you think it makes you in Warden's eyes. If 18 hours a day means you last 6 months instead of 4 months at 24/7, obviously it's worth it to cool down a bit.
Is there really any factual evidence behind it? I don't believe so. 24/7 botters have been banned, 18 hour/day botters have been banned too.
Personally, I do about 18-20 hours a day. I'm psycho and would do that by hand if I still had interest in the game, so I like to think keeping it close to my normal schedule from before is smarter.
*Shrugs*
Roll the dice, my friend. Roll the dice.