Placebo has to have end-user buy in, and I'd expect that if someone is expressly told 'these take a month to six weeks to begin to work', then a month to six weeks is when they'd start experiencing placebo.
I'm sceptical of the claim that there is no discernible benefit above and beyond placebo, but I do think there is a definite placebo effect with SSRI's in some cases, but that will require the recipient to be open to the possibility that SSRI's might have some beneficial effect.
@SabiRiver
If you don't mind me asking, it's not my field, so I'm curious. How do you go about determining a depressed state in an individual with dementia? Are we talking people who still experience periods of lucidity and can describe their own mental state? If not, how do you then ascertain that there has been an improvement in mood if the person themselves can not express this directly? I'd be concerned that if it's simply someone 'looking' cheerier, or responding more positively to certain stimuli, that in itself is no indicator at all that a depressed state existed and has been relieved by an SSRI. Again, I'm asking out of genuine curiosity and not trying to poo-poo the idea.