Drilling of long oil wells introduces the challenge of long delay time before the pump pressure has been built up to its expected level. After pump-start in long wells the generated hydraulic friction pressure requires a transient period of several minutes, due to pressure wave attenuation, before the steady state level is reached. The true pressure along long wellbores after changing the flow rate will be unknown, since the pressure normally is recorded only at the surface. Changes in pump pressure will take place not only during pump-start, but in other operations like pressure testing, managed pressure drilling, etc. Transient pressure behavior in the field was therefore investigated. For theoretical modeling we applied the water hammer theory. Theoretically estimated transient time was compared with observed transient periods in long wellbores. Model results fitted well with observations, thus emphasizing the long transient time period.