The DC voltage will be 0.707 * (Vac - 2 diode drops) however because you have a large capacitor and I assume a small load current the DC voltage could get as high as Vac- 2 diode drops as the capacitor will charge to the peak AC voltage.The voltage is shown in the schematic is 120VAC. Lastly, the AC voltage is probably 120V rms so that the 0.707 factor in the first equation is already taken into account and the peak AC voltage would be 1.414*Vac-rms.
However, remember that 120VAC wall power is not 120VAC, it can vary - see ANSI C84.1 where the AC voltage can vary from 104V to 127V rms.
Some information: The LM7812ct is a linear regulator with a 12V output that requires a 14.5V input minimum voltage and a 27V maximum voltage. Given the information above you need at least a 7 : 1 step down but not more as you won't make the minimum voltage requirement in a low line voltage case.
