The maximum yearly gain is $3000 * your marginal income tax rate. So, if your marginal tax rate is 33%, it would be $1000/year. This is because you can write off a maximum of $3000 of losses on your taxes every year.

The information you may find about tax-loss harvesting gains on the internet is usually incorrect if it comes from people trying to sell you something. For example, Betterment / Wealthfront claim that it adds an extra 1% of returns (only if you have a $100,000 portfolio and your marginal tax rate is 33%, which are the assumptions they use to get that number). On the other hand, human investment advisors are generating FUD about tax-loss harvesting[1][2], because they want to discourage people from requesting that service.

[1] http://www.cnbc.com/2014/10/24/weighing-the-pros-and-cons-of...

[2] https://www.kitces.com/blog/is-capital-loss-harvesting-overv...