If your plugs have never been changed I would do it immediately and switch dealer's as this should have been done at 37k miles (per manufacturer recommendation).
However, seems like most people have them changed by the dealer before the car hits 37k, usually somewhere in the low 30k range. Think my car had it done around 32k miles and I will be doing it again shortly when I hit 60k miles.
On a side note, quit babying the car an drive it hard! Whats the point of owing an M3 if you drive it like a grandma?