A 20$ VR game is usually (if it is linear) about 3-6 hours long.

"Superhot VR" (25$) 2-3 hours, if you are a completionist, about 6 hours.

"Star wars: Tales from the galaxy's edge" (25$) 2-3 hours, if you are a completionist, about 4 hours.

I think the longest single player VR game that exists now is "The Walking dead: saints & sinners" (40$) which is around 11-14 hours long, but it is barely linear, it is more like a fallout/far cry game but with far less interesting locations. Completionist have played it for 30 hours in total.