This phrase has two meanings. Compile time can refer to how long it takes to compile a given program (see the entry above). The compile time for a program depends on how long and complicated the program is, the speed of the computer, and the quality of the compiler.
The other meaning of compile time has to do with events that happen at the time a program is compiled, or "at compile time." A programmer may say, "Specific values are assigned to those variables at compile time."