The guidelines also place restrictions on the amount of money minors can transfer to their online gaming accounts. Gamers aged between eight and 16 years old can only top up 200 yuan ($29) per month, while the maximum amount for those between 16 and 18 will be 400 yuan ($57).
Now 90 minutes on weekdays is quite a substantial limit, as 1 match for most online games such as LOL, Dota, CS GO etc would last around 40 minutes to 1 hour. It would be reasonably easy to enforce as in China, you need real-life ID to register an account – arguably it can be bypassed by creating multiple accounts, but that may take away the aspect of grinding for one account. Something to note, most MMORPG games in China already limit how long someone can play, whereby there is a limit on experience points and rewards for playing after the time limit.
https://www.cnn.com/2019/11/06/asia/china-bans-online-games-minors-intl-hnk/index.html