Is there an incubation period for stevens-johnson syndrome?

No. Incubation period refers to the duration after which a person is transmitted a pathogen (i.e. Virus or bacteria) until the infection is apparent. Stevens-johnson syndrome (sjs) is not an infection and therefore there is no incubation period. Very rarely sjs can result from an individuals 'reaction' to an infection. More often sjs may result from antibiotics used to treat infections.
Hard to say. Steven johnson syndrome is the end manifestation of a severe skin allergic reaction. Many times the syndrome does not develop if the offending agent is removed when the early signs of a reaction appears. The time for stevens johnson syndrome to appear depends on the allergic reaction.