By definition, if you are on reserve, you are not senior. Depending on base and premerger group, 9-10 years is a line holder as a CA in NY. If you want to be in DTW and MSP then it's higher. No CA is forced to be in those senior bases, they choose to, and by extension, choose to be on reserve. If they wanted a line, they could commute to it, but QOL is king, so reserve at home is better than a line with more days off that you have to commute to.