Does it matter whether or not it is a D/s relationship or a regular vanilla one?
I think that the same feelings would still apply.

However I would not say the L word regardless as to what type of relationship I am in.
Not that I am cold hearted or unfeeling, I just find the word to be very uncomfortable for me and I could not even begin to tell you why.

Personally I find the word to be just a word, and I honestly have never been comfortable saying it to a person I am involved with. I will care very deeply for someone, and my actions do show that, but to say I love you makes me cringe.

I do love my family and tell them that, but it is an entirely different meaning all together in that respect.

Sorry I realize that it is not the answer you were looking for, but really it is not a matter of when, where, or how you say it. It is when the time feels comfortable for you.