Good question with no simple answer. A few things in play... the friction increase is the most determental item to maintaining the speed you see over the open ocean... also factors such as terrain features, elevation, wide open areas v urban areas, as well as the direction of the wind all play into how much wind speeds will decrease. Another very important factor is whether or not the low-level atmosphere is stable or not... many times the atmosphere near the coast is more unstable (warmer at the surface through the lower levels of the atmosphere) and this allows the very strong winds often found between 2 and 4 thousand feet to mix down to the surface. Many times the inland temperature profile is more stable, warms as you go higher into the 2 to 4 thousand foot level. This prevents the strong winds aloft from mixing down to the surface across inland area. Although high elevations can have an easier time seeing strong winds. The more unstable the lower atmosphere is the easier it is to get the stronger winds to the surface. Most of the time, if I don't think the inland air mass will be unstable (able to mix) I cut modeled land wind gust by 5 to 10% .