The only context I can think of that "I have enough of it." makes sense in is if there was someone offering you something you had a great deal of. Example - You are a baker and you have hundreds of bags of flour, a flour salesman approaches you with a deal on more flour but you don't need any. "Hey, would you like some of this flour at an excellent bargain?" "No thank you, I have enough of it." I've had enough of it would pertain to anything that is constant in your life; "Would you like to listen to the new beyonce album?" "No, I've had (or heard)enough of it." or if someone was upsetting you with their attitude you would throw up your arms and shout; "I've had enough of it!" and storm off. Had is using a past tense since time X until now where you are overflowing with whatever the thing may be. Have is purely a present tense where you physically have too much of something you couldn't possibly have anymore.