Answer:
Immediately after signing his soul over to the Devil.
Step-by-step explanation:
Faustus sees these words on his arm immediately after signing a deal with the devil. The words "Homo, fuge!" mean "Man, flee!" Faustus understand that this is a warning directed towards him. It is advising Faustus to "flee" from the deal and lead a life of righteousness. However, Faustus then asks "whither should I fly?" He is also aware of the fact that he cannot longer flee, because the deal has already been signed.