I've just introduced a new rat to my current one, how long should the fighting...

[Lisa]

...go on for? They've been fighting for about in hour now. My own rat has no real issue with the new younger one, it's the new one that's causing all the hassle. She keeps biting my rats tail and chasing her and provoking her into a rough and tumble. There have been a few high pitched squeaks when one took it too far, but nothing serious. How long should this go on for?

I should say, my rat is about a month and a half older than the new one, who is approx 15weeks. Both female. My own rat is just trying to avoid the fighting and looking at me to take her out.
i had their cages side by side for a week and they've had playdates in the bath. im shocked as my own rat, lily, is very much trying to avoid the conflict. it's the newbie daisy who is picking the fights. should i separate and go back to slow intros or let them work it out? and why does daisy keep biting lily's tail?? im afraid she'll really hurt her

 

[Natalie]

They will fight often until they establish who is the alpha rat. Then fighting will not be as intense, but it will still happen on a regular basis. Rats do it for fun and to establish who is boss.
So, to answer your question, the "fighting" will never end. You only need to be concerned about it if they are actually trying to kill each other. Signs of a more serious fight include puffing up, walking sideways with an arched back, standing up, hissing/huffing etc.

Just leave them in the same cage and let them work it out. The tail biting shouldn't concern you, that is just Daisy trying to pester Lily into wrestling. If it was serious, she would be going for vital areas and not the tail.

 

[Gina]

Its better to introduce slowly in a different cage, but since they're already together it'll probably be fine. Rats, like dogs "fight" pretty often to reestablish social order and dominance. Eventually, your rat will need to show the new one who's boss and the fighting should subside a lot.

 

[SlowlWormm]

The fighting could go on days, weeks. There's no telling. Here's what you need to do, because if you leave them be the results could be fatal... Try the split cage method! Get a glass panel, or something solid that the rats cant chew through, and put it in the middle of the cage. Put one on one side, and the other on the other. They'll be able to smell eachother, so getting used to eachother. And if its glass or plastic, they'll be able to see eachother too, but you dont need to. Hold it in place and makesure they cant get through. Is it wedged tightly in place and secure? You could use clips to hold it in place. Have this in place for a few hours then slowly pull it out. Are they fighting again? Put it back. Leave them with the block for a day or two, then release them again. If all else fails then ask your local pet retailer, they might know better than me.

 

[kit]

Well first off your supposed to quarantine a new rat for at least 3 weeks to make sure there's no hidden illness that could spread to the others.

Your also supposed to slowly introduce them on neutral territory until there used to each other. There's a risk that they can cause a fatal wound while fighting.