Hi,
Unfortunately there is no time estimate as to when Flex Consumption will be available in Japan East/Japan West. They have been expanding the number of regions already since they announced it was Generally Available at the end of last year.
Please see article below as well as its comments:
Azure Functions Flex Consumption is now generally available
For latest list of supported regions you can execute below command in Azure Cloud Shell:
az functionapp list-flexconsumption-locations --output table
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP