Would you help me with a question for Spanish?

With the word poco and mucho, I know that to say like how much of something you have, you have to make the poco or mucho fit with the gender of the noun and how many things there are. However, I remember my teacher correcting me once when I changed the word and said something like I just leave it as poco (or mucho) and I don't have to change it. I don't think they were talking about the number of like nouns, I think they were talking like how much time, but I'm really not sure. I was wondering if you could help me with this and tell me if there is a case where you don't change the word, thanks so much! :)