How long can a boa constrictor hold its breath?

Many factors come in to play, such as the age and size of the animal. A baby with underdeveloped lungs surely can't hold it's breath for as long as a matured specimen. Water temperature is also a factor.

It is not too out of the ordinary to observe a boa swimming underwater for several minutes at a time on just a leisurely swim, and they have been known to stay under for 20-30 minutes during mite infestations in an attempt to drown the parasites.