Online you will play 60-70 hands per hour per table. 62.5 hands/hr is a good number to use because it makes the math come out pretty.

Assume you 4-table for 8 hours a day for 5 days:
62.5 * 4 * 8 = 2000 hands/day * 5 = 10k hands/week.

Do this for 40 weeks/year = 400k hands/year. We usually talk about winrates as BB/100 hands, so that's 4000 units.

3BB/100 is sustainable at $3/$6.

So that's 4000 * 3 = 12000 BB * $6 = $72000/year.

And there are people sustaining that winrate, or higher, at much higher limits.