This is probably the most annoying question to ask, but...
Does anyone have any frame of reference of how long backorders usually take for new iphones?
I'm asking specifically about the iPhone XS Max, but just would like to know if it's typically weeks or months?
These specials and pre-orders are always one off experiences.
FWIW Pre-Ordering is generally a mugs game IMO. You lock yourself in and better deals or better availability elsewwhere are bound to come up. The iPhone 8 had major supply issues at Optus (and I think generally as Apple really didn't have enough units) but IMO the X is likely to be much more available (its essentially just a tweaked iPhone 8 which has been retired by Apple - as selling two versions of the same phone at markedly different price points makes little sense)
If you're wanting to get one fast I'd buy it outright from an Apple store if you can afford it upfront. The total cost of ownership via a plan or outright with a SIM only plan is pretty similar. But if you need to get it on a plan then I think it should be available within weeks, not months (but that's just my opinion)