Is the extra quota from this promotion permanent?

No, the increase in quota is not permanent. The extra quota is only being offered for a 12 month period upon activation of a Broadband Supplementary Plan.

Example:
A new customer who has opted for Broadband Monthly 105 will get to enjoy a monthly quota of 85GB for 12 months from the date of activation, after which their quota will revert back to 75GB.