The whole point is that God did not come from anywhere. God always was and always will be. God exists in a timeless state where the concept of what came before and what came after did not exist until God created the idea of time, and asking where he came from also makes no sense since he is the author of creation, the creator of all of the places, all “where’s” that he might be from.

Therefore asking where God came from is nonsensical, as is asking how long God has existed.