£10 MONTHLY PLAN

Just saw the pop-up offer in the app of extra data, if I switch to the rolling £10 monthly deal. But nowhere does it say how long the £10 price will last for the double data+ deal? When I switched to the £9 deal, it guaranteed 15 months at that price and I have about a year left at that price

Price certainty is vital to me, so without clarification, I won't upgrade to the £10 deal. Seems like sloppy marketing to me, when previously it was crystal clear.