As of 2018, is it possible to induce pleasure in humans by some intervention like sending electrical signals?
Yes, it is possible and has been done, typically as DBS (Deep Brain Stimulation) for various disorders: OCD and depression are a few examples where the brain are stimulated induced pleasure. (Synofzik, Schlaepfer & Fins 2012)
Regarding "How much"... How much until what? Death? Brain damage? Subjective "that's enough"? Habituation? Realize that most of what you ask is definitely unethical as an experiment (on humans, which is your focus) in this century.
For an ethics discussion I stronly suggest the aforementiond paper paper by Synofzik, Schlaepfer & Fins "How Happy Is Too Happy? Euphoria, Neuroethics, and Deep Brain Stimulation of the Nucleus Accumbens" which has been the subject of interest for popsci press (e.g. article in The Atlantic).
A few points paraphrased from the paper:
- the euphoria was a result of deep brain stimulation (DBS) treatment for OCD (obsessive-compulsive disorder) performed in the anterior limb of the internal capsule and the nucleus accumbens region (ALIC-NA).
- The pleasure experienced is voltage dependant. However, patients also reported being overstimulated when the voltage was too high and expressed that as some kind of discomfort either somewhat physical "that's too much", or more psychological concern such as that they felt "unrealistically good”.
- Habituation to the pleasure elicited by this type of DBS reportedly does occur, and pretty quickly, on the order of minutes. This is somewhat surprising given that they also mention the classic experiment of Olds (1958), in which mice kept engaging thousands of times in behavior that rewarded them with brief shocks to the brain's "pleasure center". I'm guessing there's something either in the biology or in the protocol that causes faster habituation in humans, but the paper is not clear on what that is.