Tahajjud starts from any time after the Isha prayers have been offered and one has slept for a period of time and then awoken.

The end of Tahajjud time occurs at the nautical twilight, when the time for Fajr starts. A good general rule of thumb to keep in mind is that the end of Tahajjud and the start of Fajr time begins 70-90 minutes before sunrise, however this time may vary country to country.