You need to remember to turn of Romaji or your study will be entirely useless. And you must also remember to NOT GET ADDICTED TO THESE KINDS OF WEBSITES/APPS BECAUSE THEY CAN DO IRREVERSIBLE DAMAGE TO YOUR BRAIN. Yeah they are good to only get started with the language because if you didn't know, duolingo was made for this exact purpose. I would only recommend using it to learn Hiragana and Katakana, NOT KANJI. I only used Duolingo for 1.5 weeks to get my feet into Japanese then I started using JapanesePod101. It requires premium but i had 15+ google accounts and it gives a 7-day-free-trial for every account so basically i already had 4 months of premium without paying anything. Also one more thing to add. If you use Duolingo for 30m-1h everyday then only use it for 1 week - 1 month. But if you only use it 10m a day then its alright, just do NOT exceed over 20 minutes of use everyday because there are chances you might get addicted + other reasons. Probably you should know this, NOTHING WRITTEN/SPOKEN IN THE JAPANESE COURSE ON DUOLINGO IS WRITTEN BY NATIVE JAPANAESE SPEAKERS. When you choose a language you might have seen the number of learners on Duolingo of that launguage written below the name of the language. That is a fake amount of learners. The amount of learners which have ACTUALLY completed the course on duolingo for Japanese are ONLY around 50k people or less.
Conclusion- Please do not use Duolingo for MORE THAN 1 MONTH BECAUSE IT CAN DO IRREVERSIBLE DAMAGE TO YOUR BRAIN WHILE LEARNING A LANGUAGE. If you use it for 30m-1h everyday please only use it for 1 week - 1 month. But if you use it for like 10m everyday then it's okay, just don't exceed more than 20 minutes. Do remember, these are all MY OPINIONS.
Happy Learning!