If ssri's block reuptake of serotonin, when/how often is serotonin released? since changes take a month, is it infrequent? or you just need sooo much?

It's more complicate: When a neuron fires, after it does so it will reabsorb or "reuptake" some of the serotonin it released when it fired. To inhibit this process, the neuron needs to change so it has fewer openings available at that end. The drug causes some of the openings to close, a change which takes a while. It's called "down-regulating."