This is something that you will tend to instinctively know. If you have your doubts or are nervous about having a design on you forever, then this is usually an indication that the design you have in mind might not be for you. A rule of thumb I tend to use, or at least used to use before I had so many tattoos, was to keep a design in your thoughts for at least six months, and if you get through that period and you still want to get that tattoo, then I say go get it!