Just bot for how long you feel comfortable risking your accounts, asking questions like "how long can I bot for is?" is silly as it depends a lot on several different factors which I'll try and go through now.
1) Server: Does your server have a high population? Ie. will you have a lot of people in the same places trying to farm? This can cause a lot of issues as they are more likely to report you especially if you ninja nodes... On the other hand high population can also mean big bucks for your mats.
2) Timezone: Do you play at prime time? I usually find it better to play after peak.
3) Character: No not what class you are (although that can play into consideration) but what CC/Profile combination you are running. A bad CC can make it look far too "bot like" and cause you to receive a lot of reports from players, like wise with your choice of profile.
As for overturning bans this is usually achieved by botting on an account played via VPN so you can petition Blizzard to say you were "hacked" and someone else was being the evil bot!
Hope this clarifies some things for you.
