how long can a dog be chained until it is considered inhumane?

It's usually a law that's made by counties or cities, not states. But, CA does have a statewide law - one of the few states that does.

California

2006

Health and Safety Code, Division 105, Part 6

Chapter 8. Dog Tethering

(b) No person shall tether, fasten, chain, tie, or restrain a dog, or cause a dog to be tethered, fastened, chained, tied, or restrained, to a dog house, tree, fence, or any other stationary object.

(c) Notwithstanding subdivision (b), a person may do any of the following in accordance with Section 597t of the Penal Code:

(1) Attach a dog to a running line, pulley, or trolley system. A dog shall not be tethered to the running line, pulley, or trolley system by means of a choke collar or pinch collar.

(2) Tether, fasten, chain, tie, or otherwise restrain a dog pursuant to the requirements of a camping or recreational area.

(3) Tether, fasten, chain, or tie a dog no longer than is necessary for the person to complete a temporary task that requires the dog to be restrained for a reasonable period.

(4) Tether, fasten, chain, or tie a dog while engaged in, or actively training for, an activity that is conducted pursuant to a valid license issued by the State of California if the activity for which the license is issued is associated with the use or presence of a dog. Nothing in this paragraph shall be construed to prohibit a person from restraining a dog while participating in activities or using accommodations that are reasonably associated with the licensed activity.

(5) Tether, fasten, chain, or tie a dog while actively engaged in any of the following:

(A) Conduct that is directly related to the business of shepherding or herding cattle or livestock.

(B) Conduct that is directly related to the business of cultivating agricultural products, if the restraint is reasonably necessary for the safety of the dog.

(d) A person who violates this chapter is guilty of an infraction or a misdemeanor.

(1) An infraction under this chapter is punishable upon conviction by a fine of up to two hundred fifty dollars ($250) as to each dog with respect to which a violation occurs.

(2) A misdemeanor under this chapter is punishable upon conviction by a fine of up to one thousand dollars ($1,000) as to each dog with respect to which a violation occurs, or imprisonment in a county jail for not more than six months, or both.

(3) Notwithstanding subdivision (d), animal control may issue a correction warning to a person who violates this chapter, requiring the owner to correct the violation, in lieu of an infraction or misdemeanor, unless the violation endangers the health or safety of the animal, the animal has been wounded as a result of the violation, or a correction warning has previously been issued to the individual.

(e) Nothing in this chapter shall be construed to prohibit a person from walking a dog with a hand-held leash.

http://www.unchainyourdog.org/Laws.htm

Several CA cities have additional laws on the books.

Google your county's codes. I can tell you that in Miami-Dade County (FL) it is illegal to chain your dog for any amount of time if you are not present (ie. not even 5 minutes). Google your county plus animal laws and you may find your answer.

I don't believe any time on a chain is humane (unless it's supervised play time if you don't have a fenced in yard), but as far as CA law in concerned, I have no idea.