The length of time in segregation depends almost entirely on how the incident was written up and by whom.
Seven days in already puts him past the short end of what these things typically run. If the officer on the receiving end of his outburst filed a straightforward report without escalating it, another week is a reasonable estimate. That puts him in the two-week range total, which is fairly standard for a verbal incident without any physical component.
Where it gets longer is when the officer took it personally or wrote it up in a way that added aggravating factors, things like threatening language, disrespect in front of other inmates, or any history your son has with that officer. Disciplinary hearings inside look at the full picture, and a pattern of behavior makes a difference. If this is not his first incident report at Mansfield, that history will factor into how long they keep him in segregation.
Months in the hole are not unheard of for what starts as a verbal altercation if the circumstances compound. It is worth your son keeping that in mind for when he gets out. The hole is miserable, and the fastest path back to the general population and normal privileges is staying quiet and cooperative from this point forward.
If you have not heard anything in a few more days, call the facility and ask to speak with his unit counselor for a status update. They will not tell you everything but they can confirm basic information about where he stands.