I'M GETTING IMPATIENT. How long until weed is legal nationally?

IT'S HARD TO SAY, because there are so many variables. However, it seems pretty clear that as a country and as a society, we are at a legal and cultural tipping point when it comes to marijuana. So if I have to guess, I'll be optimistic and say that federal prohibition ends in 2020.

In the first half of this decade, four states and Washington, DC, legalized pot for adult recreational use, and nearly a dozen additional states legalized pot for medical use. Today, 23 states and DC have legalized medical or recreational weed, and this year Arizona, California, Maine, and Nevada all look like good bets to ring the recreational bell. California in particular could really move things along.

Still, cannabis remains a Schedule I drug under the federal Controlled Substances Act, with a "high potential for abuse" and "no safe dose"—the same as heroin. Many states are ignoring this status, and state tax revenues have begun to accrue at astonishing levels, with no discernable uptick in criminal behavior.

Successful state experimentation has helped federal legislators warm to the idea of legal pot. In just the past few months, for example, Congress renewed a spending bill that bans federal dollars from being used to pursue state-compliant pot businesses. Also, Bernie Sanders took a short break from scolding rich people to introduce the "Ending Federal Marijuana Prohibition Act."

There are other reasons why federal prohibition could end sooner rather than later. Unless you run a private jail, I expect you will agree that the War on Drugs has been a miserable failure. According to the Drug Policy Alliance in 1980, the US had 50,000 people behind bars for drug law violations. Today, this number exceeds half a million. Drugs remain widely available and treatment resources are scarce.

Finally, the accelerated push toward national legalization is also helped by a growing awareness that the federal government has misled Americans regarding the dangers of cannabis for the last 100 years. Many people, including prominent public officials, now acknowledge smoking or having smoked pot. Many others rightly argue that the decision to use marijuana belongs with the individual, not the government.

Here's hoping the next half of this decade sees weed descend the inflection point to federal legalization. In a legal sense, this will continue through a model of states' rights and taxation; in a cultural sense, the shibboleth of marijuana as a dangerous drug will be cast further to the fringes. So let's say 2020 for the end of prohibition. And if Bernie gets elected, it might happen even faster.