it depends on the rat and where the rat came from,its history. i've had rats warm up to me the dayi got them. i've had others that took weeks of trust exercises and they are still a little cautious. each rat is different and comes from a different background. who knows howmany times your adopted has been abandoned or even if it had to fight other animals or not (i rescued one that had to fight a snake, two gerbils and a hamster before i got her--and with a bit of patience on my part she turned out to be the most affectionate, sweet and fun loving rat i've ever had). but it all takes time and patience. be slow in introducing the rats to each other and be gentle and calm with the adopted one. if s/he bites don't yell or even act angry, just pull your finger back slowly and gently,then take the rat out of the cage anyway (you may want to invest in gloves as a precaution if he's agressive as you say all the time and not just scared at the moment). once the rat settles down give it a treat to show that you're not going to hurt it and you are in fact where all the good yummy foods come from. then continue on with forced socailization to you.once that starts heading in the right direction begin rat intros slowly. as a rough estimate, based on what you have told us adn my own experience with trust training you should be able to start rat intros by around the middle of next month, maybe sooner.