You rub the inside of the lamias belly, gifting her with an extended life span.
"As long as you allow me to continue my goddess duties, I shall allow you to keep me until you die." You say with a smile.
"R-Really? Oh thank you miss!" the lamia hugs her belly with joy.

The days turn into weeks, and frankly, you are getting quite bored of this girl. You slap your forehead once you remember how long you made the girl live. Samo was already in it's modern era. Countries and empires had risen and fallen. You watched as the lamia never aged beyond 20. You squirmed on this day, desprate to get out. But even you, a goddess, could not escape the deal you made.

The heartbeat of the girl becomes a reminder of your everlasting prison. At least, that's what you thought. The girl would live for another two thousand years at the most. You could however speed up the process by getting her digested.