"Sun In" is bad for your hair actually. When you get your hair bleached at the salon, or even at home, you use products that are timed and watch its progress. Using Sun-In is essentially using heat activated bleach that STAYS in your hair way too long until you shampoo it out. Think about how long you are usually at the beach. Would you allow hair bleach to stay on your hair this long in the salon? I bet not.
It's a raw product that does nothing to protect you hair and you will have pre-mature breakage. How quickly this happens depends on the general strength of your hair. If your hair is strong, it may not happen until it grows a few more inches. Weaker hair will have breakage sooner.
I'd first recommend nature's bleach of (dilute a little first) lemon juice sprayed in your hair and be sure to condition a lot.