The details of speed restrictions are as follows: 

Speed restrictions based on the data traffic for the most recent three days 
If the total data traffic amount of WiMAX 2+ and au 4G LTE exceeds 10 GB for the most-recent three days (up to the previous day), a speed restriction which slows down the communication to 1 Mbps will be put in place starting the following day at 6 p.m. to 2 a.m. of the next-next day^*. 
Start of restriction is counted from the day the data traffic amount has exceeded 10 GB (the exceeded day is counted as the first day). 
^* For communications established before the restriction unlocking time (2 a.m.), the restriction time may be extended until 6 a.m. at the latest. In that case, speed restriction will be unlocked with a brief disconnection from the session. 

UQ Communications provides a notification service which informs users when the data traffic amount of the most recent three days has exceeded the capacity. 
If you wish to use the service, please make an application through the following page. 
UQ Communications: Data Notification Service (only in Japanese) 

Speed restriction based on your subscription plan
For Fixed Data Plan or Fixed Data with LTE Plan 
If the total data traffic amount of High-Speed Mode and High-Speed Plus Area Mode exceeds 7 GB for the month, a speed restriction which slows down the communication to 128 Kbps will be put in place until the end of the month.　

For Unlimited Data Plan or Unlimited Data with LTE Plan 
If the total data traffic amount of High-Speed Plus Area Mode with an au 4G LTE-supported device exceeds 7 GB for the month, a speed restriction which slows down the communication to 128 Kbps will be put in place until the end of the month. 
The speed restriction also applies to High-Speed Mode communication.