While I don't believe there is a "norm" due to different personalities and other factors here is what I would like if I could have it...
At least 2x per week Minimum of once per month (if I had to I could live with that) I also believe both partners should initiate. Perhaps the LD won't initiate as often, but I would say a minimum of 1/4 of the time would be nice. I am still the primary iniator in our marriage...my H still cannot seem to break down the wall that keeps him from initiating sex.
However having said that...we are now to a minimum of 1x per month, which is a great improvement over what it was...and as I said above, something I can live with if I must.