Supposedly, one of the prerequisites for becoming a host is that you educate yourself ("Playing God" et al.). Is all of this actually wasted effort, a sadistic exercise by those who maintain the pretense of some hosts being "incompatible" when all they really do is reject perfectly good hosts because there aren't enough symbionts? Or does the education amount to something that the symbionts truly find useful in their eventual joined life?
If the latter, then the answer must be 70 years of memories rather than 20 years. But the possibility does exist that "memories" or "education" are too abstract for the symbiont to retrieve from the past of the host, and only certain motoric or otherwise basic skills from that past are useful currency after joining. This merely means that the candidates are being lied to - but this we knew canonically already, and it's just a question of how much.