How long does catatonic stupor last? and what do doctors do to help them during this?

It depends: How long catatonia lasts, depends on what is causing it. In schizophrenia, sometimes low-dose IV Lorazepam can allow enough movement for the patient to eat. If it's actually rigidity from antipsychotic medications, countering with cogentin, artane, or Benadryl (diphenhydramine) may help. There are other possible causes besides schizophrenia as well, and these need to be addressed as needed. Timing varies.