>>23636271
Offhand I recall cobras tend to rear up about one third of their body length, so for a twenty foot long lamia that would be about six and a half feet.
However, lamia are adapted to move with the front of their bodies high off the ground at all times, something no real snake does. Also, thanks to many of their organs being in their humanoid portion, they can presumably devote a lot more space to muscle in their snake portion. They are also noted to have very strong, toned muscles around their humanoid hips and waist.
With that in mind I can say they can rear up a good deal more, perhaps half of their length. It may depend on how much traction/grip they can get with their snake portion to counterbalance themselves though and thus avoid tipping over.