So would it be possible to earn 1BTC a month from participating in signature campaigns using multiple accounts?
And how much time (on average) is put into it in order to earn 1BTC a month?
Yes, it's possible. Some people also buy accounts from other users with this purpose.
With 6 hero/legendary accounts posting around 50-100 post each per week you can earn 1 BTC. But generally the campaigns don't allow more than one account per user, at least in the same campaign. Probably you are safe if each account apply to different campaigns.
Also it's possible to sell the personal message and the avatar.
Uhhh that would be impossible posting 600 posts per week?? Its just too much, when i first joined a sig campaign i was like yeah im going to post 100 posts each week but then you realize that is not that easy and if you want to make constructive posts its just plain impossible to make 600 posts per week