So like how long did she live in her previous life, and how long has Rachel been in this world. The amount of time that passed for either one of them could be pretty different, with Rachel just popping into this world whereas MC had already lived out a long life after Rachel's death in the other world (not to mention the time growing up in this world for MC)
So for Rachel it's been about more or a day since she came to this world. From her perspective she was being executed and then she just suddenly appeared in the park where Lin Xiya's little sister found her.
From the perspective of Lin Xiya/Elysia it's been nineteen years since she was born into this world, and an unknown amount of time before that where she thought Rachel was dead in the old world. We're not entirely clear on when her memories came back yet, but she prioritizes her life as Lin Xiya so she's going to probably measure it from her birth regardless.
So unless Elysia died right away old world after the execution it's been at Least 20 years for Lin Xiya without Rachel, and quite possibly even more where as from Rachel's perspective its only been for however long she was held before her execution.