First, this is quoted as people will often speak this phrase. Another way to say it is: We haven't know each other for that long of a time and, um,... This is saying that the two people have known each other for short time. So you could say: We have only known each other for a short time and, um,... Notice that this sentence doesn't have the negative (not) and uses "only" to give the idea of the "short time" being a negative thing and may normally hurt a request that comes after. (If you say: We have known each other for a short time and, um... This sentence has a neutral or positive feeling and the "short time" may help the request that comes. after. This is not the sense that your original sentence has.)

"We haven't known each other for a long time" = 我们互相不认识很长时间了 "We haven't know each other for THAT long a time" = 我们互相不认识那么长时间了 Sorry if my Chinese has mistakes in it (it probably does) but I hope you get the idea

It simply means we have met recently and do not know much about each other. So she goes on to tell someone more about herself. We haven't known each other for that long but I feel like I have known you all my life. We haven't known each other that long but you seem nice

that long a time implicates an undefine space of time passed maybe for better understanding, that long of a time example: Though it hasn't really been that long a time