In your area, it probably firms up a good bit by early-mid April, once the grass is really growing, then it's using up some of the water in the soil. Otherwise, it's just evaporation which is slow this time of year. But rain or lack of it can push that date around. But your roots also aren't deep yet so the new area will stay squishy longer than other soil. But soon enough you'll be watering because its so dry.
Even if you don't water normally, I'd recommend watering some as needed this summer for the new grass. The roots are still shallow and if it goes dormant this summer, it may not come back. But nothing more fall seeding can't fix.