Why is he in solitary confinrement?
My name is Stephen R. Cohen and have practiced since 1974. I practice in Los Angeles and Orange County, CA. These answers do not create an attorney client relationship. My answers may offend I believe in telling the truth, I use common sense as well as the law. Other state's laws may differ.. There are a lot of really good attorneys on this site, I will do limited appearances which are preparation of court documents it is , less expensive. However generally I believe an attorney is better than none.
I don't believe there is any set time period, however if he is suffering health problems you need to put the facility on notice and you might want to consult with an attorney about filing a habeas motion regarding his "cruel and unusual punishment"
You may want to check with the prison and find out if it has its own internal policies about solitary confinement. Regardless of what that policy is, the prison could still be violating your father's rights if his treatment is amounting to cruel and unusual punishment. You should consult with an attorney to get further advice on this matter.