Between all the members here, some of whom ride 150 miles a week or more, others who ride 5 miles two or three times a week...figuring an "average" is kind of meaningless.
But individual averages are more practical to look at I think, so here's mine at this point in time:
Been riding about 4 months now. From Monday-Friday I usually do 2 rides of 11 miles each, plus one +/- 20 mile ride with DH. Then on Saturday and Sunday we average maybe one 35 mile ride somewhere in there, because sometimes it's raining and sometimes we are out of town or busy with non-bike obligations. So I guess that adds up to about 75 miles a week for me right now, counting rain and cancelled rides. (gee, it's more than I thought!)