When do i know my gerbil is better

ok, i used to have 2 gerbils, but one of them recently died from a cold. they were brothers and im afraid the alive one is kinda lonely. this one now also has a cold(im not sure who had it first) but we are treating him and he seems to be doing fine, i think he's gonna make it. but how do i know when he's better? because i think he's still sick(how long do they stay sick for?) but he is acting like a normal gerbil, so how do i know when he's totally well?

im asking because i want to get him a friend and when i do i want him to be better, because i don't want the new gerbil to get sick. we found out he had a cold friday, do you think it would be ok if we got another one this weekend? or is that too soon?

btw, i know the proper introduction stuff and everything, so don't say things like, " they will fight to the death!" or anything along those lines.

By a "cold," do you mean a respiratory infection? How are you treating him? Has he seen a vet? If the vet gave you medicine, then call your vet and ask. What symptoms gave you the idea that the gerbil was sick? Are the symptoms still there? If he's on antibiotics, you should finish the dosage prescribed (just like when people are on antibiotics for an illness). Nobody on the internet can decide that he's better. You see how he's acting and know what was bothering him before, and only you know what his "treatment" was, so you're on your own on this decision unless you give us more information or ask a vet (my personal recommendation).