For poolies or hopefuls, I was hired 15 months ago. At my specific seniority, I can now hold any base except DEN, CLE, or GUM.

SFO, DCA, EWR, LAX were all available to me as a new hire, either on the 737 or 320. ORD on the 737 in less than 6 months, ORD on the 320 and IAH on the 737 in under a year. I can also now hold 756 in SFO, LAX, EWR, and DCA.

Percentage-wise I'd be at 55% on the 737 in SFO, 74% on the 320, 93% on the 756.
DCA has a bigger gap, 60/85/97, EWR smaller 70/77/93, LAX 70/75/93, ORD 85/88/100, IAH 88/100/100

YMMV. It doesn't matter so much who's senior to you as it does who's junior to you. But its an easy bet that SFO, DCA and EWR will most frequently have openings junior to you, whereas ORD, IAH, and DEN will have far fewer, and the 737, being a larger fleet will have more turnover than the 320, even if neither fleet grows much.