The general rule is 15 to 20 drops on a new smoke unit and let it saturate for a few minutes before turning on the smoke. When the smoke diminishes, add another 10 to 15 drops. I usually add a few drops if the loco goes into storage for a while and will add about another 10 drops after it is taken out of long-term storage before running it.
NEVER RUN A SMOKE UNIT DRY OR WITHOUT FLUID.