New biomass maps are generated as the Sentinel-2 satellite, that provides imagery for SKYFLD, circles the Earth. How often you will see a new biomass map depends on where your field is located.
If your field lies on the equator, a new map should appear - on average - every 5 days. On lower latitudes, it takes less time - for example, in most European regions, a new map should appear every 2-3 days.
If you would like to track the exact route of the satellite, you can visit this page.
If a new map does not appear after a few days, read this article to discover the possible reason behind that.