/earliest_timestamp endpoint to find out the first available date and time for a given security at a particular interval.
The historical depth greatly depends on the instrument type and interval. Equities on a daily interval and above have a complete history since the first trading date. Whereas, on intraday intervals, it would be between a couple of months to a year.
Forex and cryptocurrencies tend to also have deep daily historical data, while the intraday is available for a year.
For the best practices and techniques of how to get the historical data see this article.