Do you mean you wanted to put chicken eggs under a muscovy duck? That's not wise, because when the duck tries to go into her water with the ducklings, the chicks are apt to follow and drown themselves. It's usually okay to have a chicken raise waterfowl, but not the other way around.
That said, if you're set on doing it, muscovy eggs take 35 days to hatch and chickens only 21, so you'd need to put the chicken eggs under her after she's already been on her own eggs for two weeks.
You can let her sit on her own eggs whenever, ducklings are very hardy and it'll take over a month for them to hatch so it'll be warmer by then anyway. But I really would steer clear of putting chicken eggs under her unless your plan is to take those when they hatch and raise them yourself.