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Some people prefer to having multiple alts, even if they are not abusing anything. Multiple projects etc. and end up having tons of them. So I too believe 3 months is too short (even though I have no alts) -- 6 months is definitely ideal. But for majority, I think 1 year is a nice sweetspot.

I think 3 months is a bit too short since I don't exactly manage the accounts that I proxy to my account frequently. Once a year is a good solid point for me to go through all my accounts and reproxy. Least I'd say is 6 months.