I suggest you do a kaplan meier plot of your data. This show if there is a problem. BTW did you order your data entry by time.? Please note there's no guarantee that your model is appropriate for this experiment. The kaplan meier plot should help you decide that issue.. A look at a good survival analysis text should be useful as well. Best wishes, David Booth

As professor David Eugene Booth said, use the plots and check the presumptions of the Cox model about your data.

if you find the cox proportional hazards assumption is violated, one alternative option is to use the restricted mean survival time (RMST)

here’s an example paper: Article Utility of Restricted Mean Survival Time Analysis for Heart ...

and here’s a tutorial for doing it in R: https://cran.r-project.org/web/packages/survRM2/vignettes/survRM2-vignette3-2.html