IMPORTANT NOTE FOR ANYONE READING:
The reps I dealt with wouldn't let ME do it, BUT others below have successfully done it -- so just keep that in mind.
This means that the current 'Big Data Plan' promos of "+3GB extra data per month for 24 months" can
The 25% off is a much better deal though, and its lifetime, so at least there is that.
The expiry I saw on the official flyer post on Reddit showed May 31st, despite many posts saying it was a "weekend only thing" (maybe only code giveaways were weekend only?).
Oh and if anyone is wondering, the 25% off is calculated only on your plan cost [ie. Big50 => $50.00] (and I believe taxes... possibly), but any MyTab\Extras stuff is added on top of that without discount applied.
Hope that helps someone,
- Binary Engineer