It really depends from person to person :)
Personally, it took me at least a month to work out that it was real.. as in I could fully trust myself to believe that I was in love (still is!) with this man and I could honestly believe that he would love me back (even if he wouldn't say it back - though mine did :] ). Sometimes it doesn't even have to be said to be understood. But if you must say it to make you feel that your relationship will survive then say it! the worst that could happen is "I'm not ready to say that yet" or a "thankyou" or even maybe a break up... though if they break up with you for that it wasn't worth the time in the first place.