I spoke to my dealer about this recently, just for future reference since I have no BMW navi currently.
She told me a maps update is available now and then, about 1-2 times per year, according to how much the roads have changed. Whether you perform the update is entirely up to yourself and costs around €250 (here in Belgium apparently).
For this reason and the high option price to begin with and the fact it has no speed cameras available, I chose no BMW navi, but instead bought a "top of the line" TomTom navi system which has lifetime free map updates (3-4 per year), traffic info, speed cameras and I can switch it between my car and work-van.
