How Long Does It Take To Litter Box Train A Kitten?

I have a 7 week old kitten and she is litter box trained, but I’ve been having a lot of trouble with her going in her litterbox. She’s always been an outside cat.

She goes once every day or two, then suddenly goes for 3-4 days without going! My mom says to not worry about it because cats can be finicky at times. Is this correct? What should I do if she keeps doing it?

Also, my mom thinks that the problem is because we don’t let her out as much as we should…she gets too use to being inside all the time and doesn’t want to go out anymore??? Please help!! Thank You!!