Unfortunately, this is one of those how long is a piece of string questions which can only be answered with,
it depends.
The problem is despite the common myths that serotonin is a *'feel good' neurotransmitter and ADs work by raising brain serotonin levels, the opposite is true in both cases. Serotonergic ADs like venlafaxine do trigger an immediate increase in synaptic and general brain serotonin levels from the very first dose which is what triggers most of the unpleasant side-effects. But
after a few weeks they force a significant down-regulation of serotonin synthesis and expression in regions of the brain linked to anxiety and depression. However, the process can revert for a while following dose increases, though side-effects severity tends to be less each time.
*neurotransmitters don't have an intrinsic property, their action is determined by the receptors they bind to, not the neurotransmitter itself.
Speak to the prescribing doctor if they become intolerable. There are ways of minimizing them with meds such as mirtazapine, the benzodiazepines (if you can get a script), or milder anxiolytics such a hydroxyzine, etc.
Most antidepressants take 4-12 weeks to kick-in from when a therapeutic dose is first taken. They work by
stimulating the growth of new brain cells in the hippocampal regions of the brain and it is the new cells and the interconnections they forge which produces the therapeutic response, not the ADs directly. Mirtazapine appears to become effective sooner for anxiety because of the sedation. It is more a sedating antihistamine than antidepressant.