Neurons are stimulated for research and therapy purposes. How much voltage (current, duration, etc) can a neuron or a group of neurons, in humans, take without being damaged?
It probably mostly depends on how you stimulate them, when we probe the ulnar nerve in practical classes with a stimulator on the skin, then 20V seems about fine. Most students will handle 20V 100 ms pulses, but some will flinch.
If you have a neuron connected directly to an electrode (in a dish), then you will get decent responses from 50-100mV square voltage steps. Individual cells will respond after 10-40 mV (depending on cell type), much more than 150-300 mV will probably be problematic (and not physiological) for single cells.
It sounds like you need to read some early electrophys, start with the squid giant axon, then Neher and Sudhof were the gods for the start of my PhD.