The holding period for non SEC-reporting companies is one year. It is 6 months for reporting companies under Rule 144. The other two responses explain some of the ways companies try to circumvent the holding period requirement.
In addition to converted debt, newly issued shares converted from preferred shares held over a year can be issued as free-trading in most cases.
Fortunately the SEC is starting to take action regarding improperly claimed 504 exemptions allowing new unregistered stock to be issued as free-trading. See the recent actions against Fairhills Capital/E-Lionheart and TJ Management.