French researcher William Tahil found in a 2008 study that there were 3.9 million metric tons of recoverable lithium deposits on the planet. A year later, consultants Gerry Clarke and Peter Harben ascertained through another study that the planet actually holds over 10-times that amount. Since then, several studies have come out, all with varying results.
We do not know for certain how much lithium exists within the Earth. As recently as 2015, it was estimated that the planet holds enough lithium reserves to maintain global production for 365 years. This, however, was at a time in which said production output equaled 37,000 tons of lithium per year.
Greentech Media estimated that by 2040, the world could see upwards of 100 Tesla sized factories to meet the growing need for electric vehicles. In that environment, based on those 2015 figures, that 365 year supply would be used up within 17 years.