Per the Merck Manual and other sources, as long as 10 days. Prodromal, "denoting the period between the appearance of initial symptoms and the full development of a rash or fever" I recently came across a claim it's up to 20 days.

In a couple of reports I read yesterday of ex-Nigeria West African clade cases, which appears to be the same source and variant family for this Western outbreak, three cases started with pustules in the groin area before they gained a lot more coverage. From memory at least two of those cases were associated with handling rodents which are thought to be the animal reservoir for the disease.