I think it depends on the Corn. My "standard" adult feeding regime is one large mouse a fortnight, but a couple of my males run to fat on that. I tend to keep them on one large mouse every three weeks and if they seem to be losing weight (which happens, but not often) then I take them back to fortnightly until they're back to their usual size. They're big lads anyway, just not that active.
I'd start with once a fortnight and then tweak it over time if you feel you need to.