You could add a ProgressMonitor when writing the product.
As computation is only done when the data is written it is sufficient to monitor this.
ProductIO.writeProduct(Product, Path, Format, ProgressMonitor)
An new PM could be created like this:
PrintPM = jpy.get_type('com.bc.ceres.core.PrintWriterProgressMonitor')
# or a more concise implementation
ConcisePM = jpy.get_type('com.bc.ceres.core.PrintWriterConciseProgressMonitor')
System = jpy.get_type('java.lang.System')
pm = PrintPM(System.out)
ProductIO.writeProduct(resultProduct, outPath, "NetCDF-CF", pm)
This should print the progress.