Two days ago, I sowed wheat, covered the seeds with dirt and gave it a good watering.
Now, however, the soil surface is starting to dry up and I wonder if a single watering is enough for the seeds to do their thing.
Should I keep watering until I see leaves or just leave them alone for a week.
If it matters, Google thinks air humidity here ranges from 40% to 99%.
Only a little water is needed. If you planted at the right depth, around 1 inch deep, the watering you gave them should be plenty to get the seeds sprouted. Depending on temperature, it will probably take 5-10 days before you see the wheat sprouting.
How much is too much? The effect of keeping the soil damp on top depends on how well it drains. In heavier clay soils, that could quickly lead to waterlogged seeds, then drowning. Also, I'm usually more comfortable aiming towards a little dry than a little wet. Adding water is simple, subtracting is hard. If you keep the soil continually damp one big rain storm can drown your plants very easy.
Leaving aside concerns of direct water damage; on a longer term constant high moisture promotes limited root growth, which means your plant uses less of the soil available to it than it might otherwise. A plant that grows it's early roots in a high moisture situation gets used to it, if things change rapidly it won't help the plant.
