Role of epigenetics in evolution and transmission of defects caused by drugs
Generally speaking, epigenetic modifications are not inherited as they are reset during embryogenesis. However, subsequent epigenetic modifications can be acquired during the period of pregnancy, which as a mechanism depends on the epigenetic state before the reset and on the physiological conditions of the mother including nutrition, health, stress, etc.. Obviously drug-intake years before pregnancy can still affect the mothers health-state.
Answering your first question: Epigenetic modifications generally cannot be directly passed, but indirect effects could theoretically pass through multiple generations without hard limit.

A major barrier to transgenerational epigenetic inheritance is germline reprogramming, during which histone variants and their modifications, as well as small RNAs and DNA methylation, are all reset. In mammals reprogramming occurs both in the germline and in the zygote immediately after fertilization. (Source: Heard &amp; Martienssen 2014 doi: 10.1016/j.cell.2014.02.045)

2: The great potato famine is said to still have epigenetic effects, generations later. 3.: Epigenetic changes cannot directly &quot;become permanent&quot; as genetic changes, although hypothetically speaking, epigenetic states can lead to viability of certain mutations that would otherwise be lethal and can therefore in theory influence evolution.