Pro Players playtime (hours)

Considering the max level is 500, and every level past 58 requires 18000 EXP to level up, and you get EXP at a standard rate of 3 per 1 second spent in match. You could reasonably assume that pro players who are level 500+ have played for: 

18000/3 = 6000 seconds per level / 60 = 100 minutes (not including placement / kills / damage EXP) per level.

100 minutes per level * 442 levels (58 levels deducted due to non-static EXP costs) = 44200 minutes / 60 = 736.6 hours to hit 500 from level 58. 

So one can reasonably assume that they've played for a MINIMUM of 737 hours. Minimum, because the level cap changes only took place on December 3rd, 2019, meaning there was a sizeable amount of time where everyone was max level and continued playing.