There are only 11 (of 2492) authors who've earned more than $1MM.
There is only one author (http://themeforest.net/user/ThemeFusion) that really looks like it's near the mark of $100k/month, with between $1-2MM, in just 15 months.
[0]Those numbers are before the network takes their cut of 30% (at that volume). So it appears that nobody on the network is making much more than $60k/month at the top. Of course, that's nothing to scoff at though. But it's certainly not short in time or effort, to achieve that.
To even think about that figure you have to be in the top 0.4% of just ThemeForest, disregarding the whole network, because then it becomes much worse.
If you want an achievable goal in any reasonable period aim for $20-40k/month, before the network takes their cut, which goes as high as 50% when you're starting out.
My recommendation would be to follow along with tptacek, and the plan he linked to above.