You immediately become immortal.

[u/CryptographerHot6198]

And the mysterious figure who gives you immortality instructs you that forever every year you remain immortal you will be given $100. And then immediately whatever you choose, you’ll be reverted back in time to when you obtained immortality, no memory of the experience, but all the money is in your bank account. How many years are you gonna do it for, and what would you do with the money afterword?

[u/KaraKalinowski]

The post isn't really clear whether you choose a number up front or choose when to end it. If the latter, I might choose to never end it. We'll see how I feel after the earth dies.