I think you're probably posting in the wrong forum here, this is about seats / where to sit on the A350...

Regardless, I think you might be a little confused so I'll try to point you in the right direction. There generally is no "let's make a deal" on CX if that's what you're asking. In general, and comments about CX's poor financial performance the last few years notwithstanding, CX attempts to protect the integrity of upper cabins by not doing predictable firesales of premium seats like the US carriers do, whether to elites or bidding games or whatnot near-in to departure. Their thinking is if they train people to think the premium cabin can be likely had by buying back of the bus and just waiting for a deal, that's what customers will do.

Since you already have an economy ticket, I think you're asking "when will I get the best deal to upgrade to PEY?" If so, unfortunately this question misses the mark... the answer to you is just "cancel your economy ticket and buy a PEY ticket outright." While CX has an "Upgrade Bid" program, it's not very widespread and certainly not something to count on appearing, and even when it is the majority of the recommended bids reported in here seem wildly outrageous and obviously something is wrong with the system (aka it'd be better off just outright buying the higher class). Anyway search around here for "Upgrade Bid" if that's your question.