Duolingo is incredibly good if you're receptive to spaced repetition learning.
The main benefit of Duolingo is the streak. Gotta keep the streak going, so I use it every single day.
Is it working? I can read French news articles for the most part, filling in the blanks from context. I have trouble listening at normal talking speeds to French youtube videos. For speaking, I'm sure I could order a croissant and get a hotel room, but beyond that...
Still, it's much better than looking for perfection and learning nothing.
Admittedly, I spend maybe 5-15 minutes per day on average and most of that I do in a rush, but the expectation still sounds fair -- reading would be relatively passive knowledge too.
Speaking from personal experience, if you’ve actually put in substantial time in the last year, you’ll find that a lot of that practice will demonstrate itself over the course of a few conversation classes with a teacher.
One thing that you will notice is that even if you can’t utilize it fluidly now, if you were to jump into more immersive methods now, you’ll realize that it’s all there under the surface - you’ll very likely make very rapid progress.
Are you reading anything? Find graded readers like from Mandarin Companion and read those (start with ones that seem too easy).
Have you learned to write any of the characters? I don’t think you need to learn to write all of them, but learning at least 50 or so got me to understand and recognize the characters better in my reading as well.
The Pleco app is a nice reader letting you look up words just by tapping on them. Also turn off pinyin in any study apps you’re using, just use characters (except for in answers to verify you’re right).
There aren't any tricks IMO. You have to practise the thing you actually want to be able to do. You wouldn't expect to get good at playing guitar by tapping an app every day, why would it help you learn a language?
I’ve used it as the way to get started and build a base of vocabulary and grammar, allowing me to comfortably jump into immersion with audio/video/speaking etc.
Duolingo is quite effective at what it advertises, it never claims it will bring you to full fluency.
How many characters has Duolingo showed you (if you can get that metric)?
Then I bought a bunch of french books for my daughter and learnt I knew nothing.
Was it really the case that you didn't know anything in the books? That would be very surprising. At the very least you'd expect to recognize vocabulary you've encountered before, even if the sentences weren't familiar. I'd have thought after 500 * 15 minutes you'd be able to figure some things out.
As it is though, even if the books were completely unfamiliar, I don't really know how you'd measure or test that Duolingo had had no impact on you. Obviously if a book has vocabulary in it that you've not encountered on Duolingo then you're not going to know what it means, but how could you know that you aren't picking up new things faster because you've worked through Duolingo first?
My experience of it has been very different. I work through Duolingo courses for languages I build software in. I've not spent anywhere close to 500 days on any language yet, but I've found I am able to work with languages better having done a few months of Duolingo courses. When I've been working on German and Spanish websites I've actually understood much more of the text than I could with websites I haven't learnt any of. I mean, I couldn't drop in to Madrid and have a chat with someone, but it definitely has made a difference.
Honestly, probably not fluent in any language except python. I probably need to put significantly more effort into learning the language than I was doing.
I hate the gendered nature of french. I love quebecois over the french academy crap.
I guess I'm not an ideal student by any means.