The deal that Faustus makes with Mephistophilis—twenty-four years of earthly power in exchange for his soul—might seem, on the face of it, like a good one. Faustus is itching to exert more power over the forces of nature and over other people. He wants the whole world to be amazed by his extraordinary knowledge, erudition, and magic skills.
But he's reached an impasse in his studies; he's dissatisfied with life and is in desperate need of a boost, the kind of boost that can only be provided by divine or diabolical intervention. As Almighty God hasn't condescended to intervene, this has created an opportunity for Satan to snatch Faustus's soul, and so he sends his dark angel Mephistophilis to Faustus to seal the deal.
The number twenty-four is significant here. In old European folktales, a day would often equal a year. Marlowe seems to have turned this convention back to front. The twenty-four years of power that Faustus has been granted will just fly by. You can see, then, just how bad a deal Faustus has made.