Mallards would fly unless you clipped their wings - one wing on each duck so that they're off balance and can't fly. Then they can't fly away. You'd have to do this every time they grew their feathers back.
No, just as the feathers regrow, which probably won't happen until they molt. Just watch for signs of molting and keep them penned when they molt, then clip the wing feathers when they have grown back in and then you can let them out again.
Ducks typically only moult their flight feathers once a year, so that is how often you will need to clip them. Just be on the look out for lots of big feathers around about this time of year. Then just clip them again once their wing feathers are fully grown back in (and there is no longer blood in the feather shafts, you must be careful of that because they can bleed out if you clip them while they still have "blood feathers").