The Netziv answers that it was not a mere fifty day process; it was much more than that. He does this by re-difining the four expressions of redemptions whereby HaShem assured Moshe that the people would be taken out of Egypt. He writes that the four expressions of redemption were four different stages of the redemption. The first stage was ve’hotzaisi; ‘I will take you out of the burden of Egypt.’ This was when Pharaoh relaxed the harshness of the slavery and repealed the edict under which Bnei Yisrael had to make their own bricks; the extra burden was removed. This stage of redemption occurred at the end of the plague of Arov (animal invasion). The next stage of redemption was ve’hitzalti; ‘I will save you from you work’. This was when Pharaoh dropped all slavery, and started respecting Bnei Yisrael to some extent; the work had stopped. This occurred after the plague of hail. Next comes ‘I will redeem you with an outstretched hand,’ which refers to our no longer being under Pharaoh’s dominion. This stage was fulfilled after the plague of the firstborns; though all slavery had ended after the hail, the Jewish People were still technically subjects of Pharaoh and under his dominion. The plague of the firstborns meant that we were no longer under Egyptian rule whatsoever; Pharaoh wanted us out. And the fourth expression of redemption was ve’lakachti; ‘I will take you for Me as a nation,’ which the Netziv reveals is a reference to the giving of the Torah, our eternal covenant with HaShem.
Now apart from the beauty of the Netziv’s explanation and the way it fits the words of the psukim impeccably, it also answers the Netziv’s original question. For if one does the maths, it turns out that from the first stage of redemption to Mattan Torah there was at least eight months. And from the first plague until Mattan Torah there was well over a year. So it was not just the mere amount of fifty days for Bnei Yisrael to turn themselves from slaves to supermen (without the phone box), but rather a solid period of eight to fourteen months to gear themselves up and raise themselves up to mental and spiritual heights.
Indeed, this explanation of the Netziv also reveals why there are four cups on seder night and not one; surely there’s only one Exodus (terrace chant!)? The answer is that since each expression of redemption was a different stage in the redemption (and not merely four assurances that the same event would take place), each stage is worthy celebrating, reliving, and treasuring in and of itself. And let’s drink to that!