You're looking for an answer, there is no answer. Just guesses, the longer the safer.
Why were you suddenly so concerned? Was your SS reviewed?
If you want really safe, here is my advice:
1) Have one IP per account and never switch between those, don't use public proxies.
2) Never cheat
3) Never trade items between accounts
4) Never accept deals that seem too good to be true
5) Never gift/accept gifts
6) Don't click links to NeoPets from cheat sites (referrer address)
Any thing else and you may be banned, even months after the incident, without warning, any accounts that have ever shared the IP, traded, gifted, etc could also be at risk.
That's pretty much the best advice I can give, the more you do these actions and the more often the more risk you give to your accounts.