Assuming the time line is stable I would say any time in before a global catastrophic event such as the K-T dinosaur extinction event since any meddling there will be likely erased by the destruction. The latest cut-off point should be when the first true humans appeared, around 2-6 million years ago.
However, how does your novel account for not disrupting the timeline? Is the model a parallel universe, i.e. going back in time creates a new reality branch? If such is the case, then there are ethical issues for the putative time traveller.