The [Nominatim Usage Policy](http://wiki.openstreetmap.org/wiki/Nominatim_usage_policy) for OSM's own Nominatim instance says
> As a general rule, bulk geocoding of larger amounts of data is not encouraged. If you have regular
> geocoding tasks, please, look into alternatives below. Smaller one-time bulk tasks may be
> permissible, if these additional rules are followed
>
> * limit your requests to a single thread
>
> * limited to 1 machine only, no distributed scripts (including multiple Amazon EC2 instances or similar)
>
> * Results must be cached on your side. Clients sending repeatedly the same query may be classified as faulty and blocked.
If you say that speed is not critical for you, then to be absolutely safe I'd insert a longer pause between requests - say, 5 seconds - which would take your total run time to about 1.5 days, and you could be sure not to tax OSM's servers too much.
You could also look into [MapQuest's Nominatim Server](http://developer.mapquest.com/web/products/open/nominatim) the usage policy of which is somewhat more relaxed than OpenStreetMap's (they have bigger machines).machines).
