What is the lifetime of the synapse in the mature brain? I know there is synaptic plasticity in the mature brain. There is a great number of plasticity mechanisms like LTD and LTP, acting on different time scales. Apart from the synaptic weight change, there is constant rewiring. New spines are being born, those who 
find a synaptic bouton can even expel the previously connected spine to that bouton.

What I don't know is the specific time constant associated with the rewiring. I also don't know the net effect of this plasticity. What is the average lifetime of the synapse? 
What percent of the synapses are rewired, say, in a day? The motivation behind my question is to understand, how volatile is the brain and how fast does it change. For example, are my distant memories of childhood written in the synapses that never changed since their creation or have they been already rewritten several times?

I understand that the answer depends on the species, brain area and type of activity.
Ideally it would be , say, human broca's area in normal lifestyle. But I'll be happy to know answers for other circumstances as well.
In adult mouse cortex, synaptic spines can last anywhere from days to months.

I'm not sure that any such work has been done on human Broca's area, but keep in mind that many synapses are experience dependent.  One study found that fear conditioning and fear extinction lead, respectively, to synapse formation and elimination.  So the lifetime of a synapse could depend largely on environmental conditions.