For me, it depends on how big the "spot" is and whether it's a specific "spot" or a trail or bank full of them. Each "spot" will likely get several casts, not only from different angles but also at different depths (and maybe even different speeds).

I used to think that I was spending no more than 30mins at a spot/area before moving, if I hadn't caught any decent fish from it ... when, in reality, I was actually there two to three times longer than that. I'm a line watcher, not a clock watcher