Depends a bit how dirty the old stuff is and how scrupulous you want to be. 1L should be more than adequate for a simple change I'd suggest. If it's been neglected for years you might want to purge more through, I've seen old cars with what looked like cold coffee in the brake system which needed the pedal to be pushed down in order to shift it, my Mityvac wouldn't get it to flow through.
Personally I work on the little and often type of principle. Change fluid every couple of years and just do a simple change, empty the reservoir first using a vacuum type pump (Mityvac), top up, bleed through maybe 100mls or so at each wheel (and clutch if it is combined) and leave it at that. I don't bother with ABS circuit bleeding etc, working on the principle that as long as the bulk is clean and fresh every couple of years then any "dirty" residue will get diluted and not become harmful. Just make sure you don't get any air drawn into the system by running the reservoir too low.