I did the math, based on the screen shot for the Weekly Challenge menu that showed how much it takes to unlock a level of the Battlepass, which is 10k EXP per level, from what it seems.
Since, 10k EXP is half of how much is needed to level up the boarder in OW1, 55 BP levels is equivalent to about 27 border levels.
So, about 27 border levels worth of EXP is needed to unlock a new hero from just grinding.
And if you imagine how long it usually takes for you to level up 27 border levels, try to imagine how much faster would it go, when completely the Weekly challenges for 5k points each.
If the player does end up completing 11 Weekly Challenges in a week, that is 5 & a half border levels, on top of the normal rate of playing, when grinding 27 border levels (assuming the amount of EXP you gain from playing a match in OW2 is the same as OW1).
And that’s just the BP EXP you get from Weekly Challenges.
Who knows how much more EXP you get from the Daily, Seasonal, & Competitive challenges.
(There are tabs for all of those categories in the Weekly Challenge screenshot.)
After taking all of that into consideration, I feel like the normal rate of achieving 27 border levels in OW1, which is about 55 BP levels, will be much more sped up than what we perceive to take a long time.
And that’s just due to earning EXP from challenges.
We still get the extra 20% EXP boost from playing matches as a group.
Personally, I almost always play solo & get through 25 border levels without much thought, within 1 or 2 weeks.
And those are usually my dead weeks, when I play the least amount of OW1.
But it will differ, on a player to player basis.