LGA/PHX/LAX 320 have all gone to new hires in class this year.
DC/MIA 320 have gone pretty junior lately. A few months seniority.
Everything else expect pretty much 6mo - 2 yrs or more, moving target and hard to say really. CLT/DFW/PHL are the most senior of the 320 domiciles to hold.
737 is typically more junior and quicker to hold in nearly every base where they co-exist with the Airbus. It's always changing and hard to say with any certainty what a new hire can expect, just don't expect DFW or CLT offered to a new hire class. That is pretty much a certainty to not happen any time soon now that the S80 training has ceased.
Basically if you want LGA, PHL, LAX or MIA you are pretty lucky because you can get any of those right away or within months typically (maybe not on Airbus but on something). PHX hasn't been offered again for months I don't think but was offered earlier this year a few times to new hires.