Indeed this research could only be undertaken over a long time period of time without any disturbances of water and air. (sciencedaily.com)
If she cries for long periods of time for seemingly no apparent reason, it can be very stressful. (littleremedies.com)
You can set the length of each study session; you could complete the course in one day or study for short periods of time over several days. (nationalhighwaysafetyadministration.com)