So sometimes it takes couple of seconds, sometimes couple of hours? Maybe there's some approximate range.

C++ compile times heavily depend on the code that is being compiled and the build system that is compiling it. Code that doesn't touch STL or boost or do much metaprogramming of its own is going to compile extremely fast (especially when compiled in parallel). Code that pulls in half of boost isn't going to compile so quick without tweaking the build settings. Due to how variable compilation times are, I don't think the range you provided is that unreasonable. Build time is something that changes on a project to project basis.