Examples: https://www.google.com/amp/s/amp.reddit.com/r/Vive/comments/...
https://www.google.com/amp/s/amp.reddit.com/r/PSVR/comments/...
https://www.google.com/amp/s/amp.reddit.com/r/oculus/comment...
Very, very few actual developers in those boards.
"Getting your VR legs" is definitely a real thing.
More concretely, there are a number of different artifacts of current VR systems that will cause different side effects, depending on how long you expose yourself to them. Unfortunately, the common vernacular lumps all of those issues under the title of "simulator sickness". While simulator sickness is a well-known problem, it is by far not the only one, and if you're experiencing nausea or discomfort during a VR experience, you should not just assume it's "simulator sickness" and try to "power through".
IPD mismatch is your eye telling you things are at a different distance from where they feel when grabbing or traveling to them. In AR, this can make 3D objects look like they are swimming, yet also remaining stationary, which is disconcerting but ultimately not a big deal. In VR, it can make you feel kind of dizzy, a bit like you're swimming, because the effect is very similar to viewing things through water, or a glass of high refractive index. If you've ever had to switch between contacts and thick glasses, then yes, it is trainable.
Some people find bad depth cues to be nausea inducing. There is evidence that the brain weights different depth cues more or less strongly depending on one's overall hormonal balance. People with higher testosterone levels judge depth through motion. People with higher estrogen levels judge depth through relative object size and shadowing. This isn't just "men vs. women" as there are any number of reasons your endocrine system could be off the normal distribution, but it is why a lot of women feel very uneasy immediately upon putting on a headset and why men tend to not like monoscopic 360 content. The effect is so strong for some women that I've never seen anyone even attempt to power through it. The best solution is to stop using that particular app and start using one with better graphics.
But most importantly, low framerate can cause a violent, persistent nausea and headaches, within seconds. "simulator sickness" on the other hand, takes extended exposure to develop. Higher framerates are always better, but 75Hz seems to cover about ~85% of people, 90Hz seems to cover ~95% of people A VR experience, and 120Hz covers ~99%, which at that point is actually better numbers than people going to the movies watching action movies. This is one of the reasons I found it so disappointing that the Oculus Quest was set to 72Hz; we should be going up, not down. If the application you are using is not running a very high framerate, you're in for an extremely bad time, with it only getting worse the more you expose yourself to it. You absolutely must not try to power through low framerate.
So this is why I say "don't try to power through sim sickness". It might not be sim sickness. Take breaks whenever you feel bad. Whatever "VR legs" are, if it's just the proprioception or IPD mismatch, you'll eventually get there even if you don't "power through". But there are plenty of other issues that you should definitely not try to "power through".