This will be very difficult to even give a rough estimate. If the pipe is buried in very porous sand, such that the effluent can easily percolate through the sand away from the leak, you could flow quite a lot of liquid. OTOH, if the pipe is buried in very fine clay, you might not flow much liquid at all.
You can give an absolute upper bound for the leak velocity as C*(pressure * 2/density)^0.5 (make sure your units are consistent, i.e. density in kg/m^3, pressure in Pascals (N/m^2, which is actually kgm/(s^2m^2)), and velocity in m/s), and C is the discharge coefficient for a hole in the side of the pipe, maybe 0.7). But the values for a buried pipe will be far, far below that.