A few seconds, because no damage occurs to begin with.
There are many reports of marijuana causing brain cell damage, which in turn leads to memory loss, cognitive impairment and difficulties in learning. There is no significant drop or increase in IQ points (which is an absolutely ridiculous, scientifically unsupported and inaccurate means of measuring intelligence to begin with) between twins when one consumes marijuana and another does not, there is no difference in GPAs among marijuana consumers and non-consumers, and some longitudinal studies of college students, after controlling for other factors, actually found marijuana users to score higher on tests than their non-consumer peers. One study that found lower average grades for marijuana consumers failed to identify a causal relationship and concluded that both phenomena (consuming marijuana and having low grades) were part of complex, inter-related social and emotional problems. One wasn’t the cause or effect of another.
The original study to make this claim reported in postmortem examinations of rhesus monkeys exposed to high quantities of THC to have abnormalities in the hippocampus, a cortical brain region known to play an important role in learning and memory, and thus concluded there must be negative side effects. They further “confirmed” this study by finding similar brain changes in rodents, who were given up to 200 times the psychoactive dose in humans. For reference, this amount in rats to produce minor abnormalities is comparative to a human smoking 30,000 joints and being reported as having some lingering brain dysfunction. Further studies found no significant brain abnormalities in rodents with 100 times the psychoactive dose in humans. In 2006, the Nathan S. Kline Institute for Psychiatric Research and the New York University School of Medicine scanned the brains of 10 individuals who consumed cannabis daily for one or more years in their adolescence, and 10 control subjects who had not, using MRI methods. There was no evidence of cerebral atrophy or loss of white matter integrity, and the researchers concluded that frequent cannabis use is unlikely to be neurotoxic to the normal developing brain. Plenty of other studies have been conducted using similar methods and found no long-term negative health effects when looking at their brains.
Clinical research has continually reaffirmed the opposite of these brain damage claims as well; the phytocannabinoids found inside cannabis actually have potent neurogenerative and neuroprotective properties. Compounds like THC and CBD play a role in helping the brain to build new neurons and then protect them from neurodegenerative diseases or injuries caused by stroke, concussions or head trauma.
More recent studies have been finding measurable differences in brain matter, particularly in the amygdala and hippocampus, among chronic cannabis users and non-users. The issues with a lot of these claims is their inconsistency; no discernible pattern has been found in what kind of brain changes occur, whether or not these changes are inherently negative, positive or neutral, and how researchers define “chronic” cannabis use seems to differ drastically from study to study, with some considering “multiple times a day” to be chronic, and others considering “daily” as chronic. Despite these inconsistencies, studies have continuously claimed to observe cannabis users having difficulty with attention, concentration, decision-making, risk-taking, impulsivity, inhibition, working memory and verbal fluency. Much of the medical world, and certainly much of the cannabis industry, sees these examples as outliers or victims of the very subjective experiences one can have from cannabis use. The majority of these studies use sample sizes of around 5 people, did not specify dosage amounts or lengths of abstinence and did not control for outlying factors. Some people can operate fine on cannabis, and some people just can’t.
There is no consistent evidence that marijuana users, even those of decades or more and of any age, suffer permanent brain impairment in any regard. Numerous studies comparing chronic marijuana users with non-user controls have found no significant differences in learning, memory recall or other cognitive functions when controlling for other factors.