Christopher Kierkus (dpreview, Panasonic forum, 15th February 2004)...
If the light is low enough that the shutter speed is 1/4 - 1s the autofocus seems to have trouble but you can still manually focus. At 1s - 2s the EVF becomes so dark that manual focus gets very difficult and you have a real problem. At 2s + the EVF is basically black and no focusing is possible.
Of course this varies, if there is a small light source in an otherwise dark area your shutter might be very slow but you can still focus on the light source, but the above number hold as rules of thumb.