The Jewish people were in Mitzrayim for 210 years. When Yosef lived, Benei Yisrael were the elite above the Egyptians; after he died, they equaled the Egyptians; after his brothers died, they declined, becoming a despised minority; after the whole generation died, their status was lowered further the Egyptians enslaved them (Ohr haChaim).
Though in the brit bein habetarim Hashem told Avraham that his descendants would be oppressed in a foreign land for four hundred years This period was counted from the birth of Yitzchak. The period of slavery was reduced by (i) the labor’s harshness; (ii) Bnei Yisrael’s miraculous population increase, resulting in a huge number of slaves (Mishneh laMelech). The real years of slavery and suffering lasted 86 years – after Miriam was born (Rashi – Shir haShirim 2:11).
When Moshe was 79 years old, Hashem appeared to him at the burning bush Shemot (3:9)
And now, behold, the cry of the people of Israel has come to me, and I have also seen the oppression with which the Egyptians oppress them. Therefore, come now, and I will send you to Pharaoh, so that you may bring My people, the sons of Israel, out of Egypt.
Eighty six years of crying and prayer were finally answered – What took so long for the prayers and cries to trigger G-d’s response?
The amazing answer is that just as in nuclear physics a critical mass must be arrived at to trigger a nuclear explosion so too our prayers, suffering and mitzvot and good deeds must add up to a spiritual critical mass, according to tikkunim in the worlds above, to obtain a physical response.
Obviously we haven’t quite reached this critical mass for the final redemption as yet, but we are very close -KEEP PRAYING!!!