RPMs before valve float/damage?

How many rpms will a 12v tirn with 60# springs before the valves float or hit a piston?
I work with a guy who was asking me how fast my truck would go, and I said 'till it floats the valves. He asked again how fast that was, and I said I dunno, the speedo only goes to 120. I dont have gps or I would check it off of that, but I can figure it off of rpms. But all I know is its faster than I should be going, and it pulls hard till it hits a brick wall, and the engine changes tone and sounds like the valves are floating.
 
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