How long in quarantine?

A few months ago a friend brought two axolotls because she heard me raving about them so much. Last night one of them died, I suspect it was the water quality (I'm going to test it tomorrow for her) but she's decided to give me her surviving wild type because she's scared she'll make that one sick too. I thi she's probably doing the right thing, and I have a spare tank for it to go in but in the next week or two I'm expecting the pet store to call me and let me know if my new white axolotl is in. My main worry is the only other tank I have is a 5 gal and I don't know how big they're gonna be (5 gal would only really do for a small Julie for a few weeks at best anyway). So my question is how long should each of them be quarantined? I'm more worried about my friends axie, it's probably the water but she might be sick. I haven't had any problems with that particular pet shop (although its better to be safe than sorry)

I've heard that 30 days is a good medium, though I'm not entirely sure why. We quarantined Galaxy for a week (he came from a store) and that was fine, but I think 30 days is better if you have concerns. Hope that helps!