(A) If someone visits New York City or Boston for a few weeks and works out of a hotel or Airbnb, they are legally required to file a New York or Massachusetts tax return and pay New York or Massachusetts tax on the income that they earned there. That's the logic that extends to other states. It's true that some people will break the law. We do not advise you to break the law.
(B) Any wiggle room that might have existed disappears when a visit becomes "a really really long visit." You're not talking about "a few weeks." You seem to have been in California long enough to be considered a California resident for income tax purposes. Read the FTB publication.
(C) The California FTB is very aggressive about collecting taxes. Their starting assumption is that you owe California tax. They will not give you any wiggle room. If your residence situation is unclear, they will probably say you are a resident, unless you can prove otherwise. But you have to pay California tax on your California income, whether you are a resident or not.
(D) I again suggest that you consult a tax professional in California.
