Is there an incubation period or can I take it right before we encounter them for maximum coverage? A month at sea really isn't much time, so I want my immune system tip-top before we fight these things.
That potion is not magical, so it will not taken effect immediately. The alchemists say they recommend at least an hour before, but best efficacy is 24 hours prior.
The shorter duration ones take effect more quickly, but last much less. For example, one round before would last eight hours and cost 250gc.