The short answer without knowing more about your case is up to six months in custody.
The diagnosis should have no bearing on what the maximum penalty is whatsoever. It can help in petitioning the court or the prosecutor for a "sweeter" deal however. Your diagnosis could result in a resolution that includes treatment rather than custody.
In California courts, petty theft which is defined as theft under $400.00 is often let go without jail time and a short period of probation. If someone has a prior offense that is theft related, there will be more effort required to keep the sentence light.
Sentencing in Federal Court however is based on a set of tables that take into account criminal history and the category of the crime. Often a significant jail sentence can be involved in what is otherwise considered a minor state offense. I've seen six month sentences handed down for a DUI on Federal land that would have only resulted in two days custody had it been on State land.