The ability of our body to function is directly linked to our ability to breathe. In the world of BDSM, breath control is considered 'Edge Play', a practice where the bottom literally puts his life in the top's hands. Giving up control plays a big role in the pleasure gained from this experience.
Trust is the big factor in this practice, as the bottom can become extremely vulnerable. This exchange of vulnerability and control is the gateway to deriving pleasure out of risk for the bottom and top.
If not done properly, this practice could result in serious consequences such as paralysis, heart damage, irreparable brain damage and death. Keep in mind that oxygen deprivation can cause permanent brain damage.
It’s hard to say what is safe and for how long, but this practice is not recommended for people with heart or respiratory disease. The most important piece of advice we can give, if you are planning to practice breath control by yourself is: DO NOT DO IT!