The short answer is, there is no time limit for crew duty/flight time with 14CFR91 ops.
These are private ops, therefore unregulated.
As it should be in my considered opinion.
However, 14CFR91 subpart K regulates fractioinal ops, and there is a time limit on these flights, as I recall.
Most all reputable companies operating long range business jets do have an ops manual however, and therein are stated the crew duty/flight time limits....generally quite reasonable.