Fox et al used "diabetes duration" and "duration of diabetes":
Fox, C. S., Sullivan, L., D’Agostino, R. B., & Wilson, P. W. (2004). The significant effect of diabetes duration on coronary heart disease mortality: the Framingham Heart Study. Diabetes care, 27(3), 704-708.
so it seems you could substitute "duration" in your sentence. More generally, "disease duration" is common.
Trogdon & Hylands used "time since diagnosis":
Trogdon, J. G., & Hylands, T. (2008). Nationally representative medical costs of diabetes by time since diagnosis. Diabetes care, 31(12), 2307-2311.
(in your sentence you could easily use units with this, too, as in "Jane was diagnosed with diabetes when she was 42, and now she's 55, 13 years since diabetes diagnosis."