When v12(t) reaches in amplitude value of VD+VZ=6.8V, DZ1 will start conducting in forward direction DZ2 will start conducting in reverse direction. While conducting, they will provide constant voltage drop on R, 6.8V.
That's what I know
At first glance the answer looks obvious. But to by honest the more I think about it, the more confused I get. So I don't know how you can find this output voltage.
I got the attached results in a simulation. There was NO VD in your diagram. I did set an initial charge on the capacitors to kickstart the oscillations. Post your multisim.