Kind of like simple answers for something like this ... I tend to think each Oath cuts Lifespan by x%, thus the more you take the shorter your lifespan.

If you assume you cut 25% off for each Oath you have a woman who would have lived 700 years would live about 525 years with 1 Oath, but only 394 with 2 Oaths and about 295 with 3 Oaths and perhaps only 220 with a 4th and so on...

Now if the same woman takes the Oaths at 300 I tend to think it would cut her remaining lifespan thus she would live for 168 additional years (400 years left to live -25% for each oath). But who knows!