Translation:How many hours do they work?
Nan ji 何時 = At what time?
Nan ji kan 何時間 = How many hours? (although you could interpret that as how much time? also as you mentioned).
It's not "how many times."
何を買いますか (nani o kaimasu ka)
What will you buy?
It's a little more complicated to ask "how many will you buy", because the questions changes according to what's being bought. 何 combines with the counter.
本を何冊買いますか (hon o nansatsu kaimasu ka)
How many books will you buy?
何個買いますか (nanko kaimasu ka)
How many small objects will you buy?
Yuyitou!! You did not understand the comment you are criticizing. That member of this discussion had already correctly and helpfully "broken down" the sentance to answer a request. By then adding "ka=?" the helpful person was EXPLAINING the meaning of "ka", which she/he had forgotten to add to her/his previous comment. I inwardly had the same reaction as you did at first, but upon looking at it AGAIN, fortunately understood what it actually meant.
The "type what you hear" questions are automatically generated from the lesson sentences. In most languages, there's only one way to type what you hear, so even if the exercise is automatically generated, there should only be one correct answer. Unfortunately with Japanese, there are multiple correct ways to type the same thing, but only the top answer can be accepted. Most users recommend to other users to use the word bank for those questions or skip them entirely until Duolingo can come up with some kind of fix for the problem.
When I'm paid by the hour, and i have conversations with my coworkers, this question sometimes comes up, with the answer, "oh, they're full time," or "4 hours Tuesday and Thursday and 6 on Saturday." Another common question that comes up is, "how many hours did you get this week?" I'm sure it's different in some senses, and it's a bit obscure, but not unheard of.
DwiArya, are you confused about the Japanese or the English? It seems the Japanese has been explained well in this discussion, so maybe if you go back and check earlier comments, it will help. If you are asking about the English translation, it's true that we might commonly say "How long do they work?", but in this case it would not be specific enough to translate this particular question. "Length" of work time could be answered in hours, days, weeks, months or even years.