Your Options:
When you purchase your eSIM, you will be given the option of an 8 Day, 16 Day or a 31 Day version of the Japan Tourist eSIM.
Your Japan Tourist eSIM will be ready for use as soon as you activate it and so your 8, 16 or 31 day service duration will just begin from the day that you activate the eSIM.
As this is a pre-paid service, you will only be charged for the initial purchase and no further charges can be incurred.