What is the test for premeditation in a murder case?

"The true test is not the duration of time as much as it is the extent of the reflection. Thoughts may follow each other with great rapidity and cold, calculated judgment may be arrived at quickly, but the express requirement for a concurrence of deliberation and premeditation excludes ... those homicides ... which are the result of mere unconsidered or rash impulse hastily executed." (People v. Thomas (1945) 25 Cal.2d 880, 900-901, 156 P.2d 7.)