how long should i sit with a tattoo idea before getting it?

in my life ive only gotten 2 pretty decently sized tattoos... i dont regret either one, but lately ive been wanting to get a giant one in a pretty big area, ive been thinking on it about a week... and im wondering how long i should wait before going through with it? is there a general time frame that i should sit with the idea? how do i know if the tattoo is right for me or not?