How long did Israel's slavery last in Egypt?

Many think 430 years to the text in Ex 12:40,41. But God gave the key to understand this text all the way back in Gen 15:16 and the Bible itself provides the evidence in Ex 6:15-26 that it has been half that long.
The fourth generation would return (Gen. 15:16). And Joseph saw from his sons the 3rd generation - his great-great-grandchildren - and he was only 110 years old (Gen. 50:22). It could never be 430 years from when Jacob went to Egypt because of the famine. Therefore, the beginning of those 430 years must be earlier in time. It was the promise to Abraham in Gen 15 that was fulfilled with the exodus.

Here lined up at a glance ...
Joseph 17 yrs. - Gen 37: 2.
Joseph 30 yrs. - Gen. 41:46.
Joseph 36 yrs. - Gen 41:50, in the sixth fat year his twin (?) Sons were born: Manasseh and Ephraim.
Joseph 39 yrs. - Gen 45:11, after the second meager years he caused his family to come across.
Joseph was 56 yrs. - Gen 47:28, when Jacob died (Jacob lived in Egypt for 17 years).
Joseph 110 yrs. - Gen. 50:22, 26 and in Gen. 50:23: Joseph saw of Ephraim the third generation, and the sons of Makir, the son of Manasseh, were born on his knees.
Ex 1:6: the whole family of the sons of Jacob died. Another pharaoh whom Joseph did not know came; it must have been that this crown prince was born after Joseph and all his brothers had died. Let that crown prince be 40 when he came on the throne. After that he will be fighting the people for an unnamed period - cannot reasonably be more than 20 years..
Ex 2:1 Aaron, Miriam and Moses are born. Aaron was the 3rd generation of Levi, the brother of Joseph.
Ex 7:7 - Moses was 80 when he led the people, Aaron 83.

Added: 83 + 20 + 40 + 110 -39 = approx. 214 years.

From Ex 6:15-26 we can clearly extract four generations:
15 Levi, brother of Joseph, had three sons: Gerson, Kehat, and Merari - the 1st generation

16 Gerson's sons were Libni and Simi
17 Kehats sons: Amram, Jishar, Chebron and Uzziel.
18 Merari's sons: Machli and Musi. - the 2nd generation

19 Amram married Jokebed, and received Aaron and Moses.
20 Jishar's sons: Korah, Nepheg and Zikri (Korah, was the one who revolted against Moses and Aaron (Num 16). He was their full cousin)
21 Uzziel's sons: Misael, Elsafan and Sitri. - the 3rd generqation

22 Aaron married Eliseba, and received Nadab, Abihu, Eleazar, and Itamar - the 4th generation.

Nadab and Abihu died before the Lord. Eleazar and Itamar then served together with their father (Numbers 3:2-4). Eleazar succeeded Aaron: Num 20:25, 28; Deut 10:6

The fact that Eleazar crossed the border into the promised land, means that he was not yet 20 at the time of the rebellion of the ten spies, because everyone from 20 and older died in the desert (Num 14:29).

Now back to th eoriginal promise
The promise to Abraham was that not earlier than the 4th generation would set foot in Canaan again. Eleazar belonged to the 4th generation of Levi, the brother of Joseph.

24 Eleazar the son of Aaron took one of Putiel's daughters to a wife, and beget Phinehas, evidently in the wilderness - the 5th generation. Pinehas was the priest on the other side of the border who pierced the whoring Israelite and his Moabite sweetheart with a spear (Num 25:7,8).

See, ...
the Bible always has the last word ....