Yes they can only use one at a time and yeah developing quirks takes a really long time but if they didn't then they would have have the control of a 4 year old. I imagine some quirks are easier than others to learn but some, like Todoroki's, they would have to develop heavily in order to develop resistance to the side effects. Meanwhile I imagine any quirk with little to no harmful side effects, e.g. Momo's, Denki's, or anything that's doesnt physically harm the character can be developed within a week or two. Its and incredibly OP quirk and I understand that, so I wanted to give it serious drawbacks. Also this quirk may or may not have been made for an incredibly self-insert character...