Does anyone have any rough idea how long my car is likely to run for with a buggered altenator??

Last time it packed in it was gradual and i got away with it for a couple of days as the altenator was partly charging.
I cant quite remember though how far i drove once the altenator had stopped completely, but it was winter and dark so had to have lights and heater on.
Today it appears to have packed up overnight. Ive had an electric battery charger on it and the voltmeter on the dash is reading just above the lower 'normal range' limit.
Will i get away with driving 20miles or so???
Hope this isnt a 'how long is a piece of string' question.

Also, my altenator belt has been screeching for a month or so due to oil leaking onto it. It usually shuts up after a minute or so, but this morning it didnt screech at all. Is it possible that its got so oiled up that its just not driving the altenator at all??
(cylinder head nuts were loose after having a new rocker gasket 6months ago and oil has pissed all into the altenator again so chances are its buggered)

thanks
Ellie