how many days notice does a bank have to give before payment is due when they increase it?

Typically the bank cannot change or increase the trial modification payment because this is a precursor for approval for a full modification.  In terms of the bank, if your bank is regulated by your state, contact the State's financial institution's department.  If your bank is a national bank, contact the Office of the Comptroller of the Currency.  If your bank is regulated as a savings bank, contact the Office of Thrift Supervision.

You should review your paperwork to determine if this increase is actually now part of the actual modification, and not part of the trial or if it is actually an unfair increase without written approval on a contract by you (which by the way the bank would need to increase these payments based on the Statute of Frauds).