If a neurologist has diagnosed someone with pseudo dementia and he is being treated with celexa, (citalopram) when should symptoms go away?

They may not. Celexa (citalopram) is an anti-depressant, and it may help if the patient has a depression which is masquerading as a dementia, which happens occasionally. Otherwise Celexa (citalopram) does not really improve memory and thinking.