They were hiring right into MIA 75/76 for a while a couple years ago. It goes in cycles. But I'd say you can hold MIA 75/76 within 6 months to 1 year from new hire. LGA 75/76 goes a little more senior but still within a couple years. DFW, ORD, LAX are all a lot more senior and it would take a few years. But keep in mind, with the 75/76 draw down this is all subject to change.
This is just for 75/76 (group 3.) 777/787 (group 4 flying) is a whole different animal and takes much longer, several years.