We hear a lot about "tax loss harvesting" and the benefits thereof. I could never get a solid picture of how it would work in real life over a long term of years. So I make a spreadsheet to examine it,

The idea is to occasionally sell a stock or ETF you like but which is currently sitting at a loss, to book the taxable loss. Simultaneously, use than money to buy another holding which is quite equivalent. Such as the SPY-VOO pair. (Or the same ETF at different brokers and figure it won't get noticed.)
You must not re-buy the one you sold in the next 30 days to avoid the wash-sale rule.

Some simplifying assumptions in the spreadsheet:
* The two members of the pair so closely track each other that you can do all the math using the price data of just one ETF.
* The sell decision is made at the end of every month at the close.
* All trades are made at the open of the next month (i.e., the next day).
* No friction on the trades.
* Dividends are ignored.
* The minimum holding period is 2 months. That is, no trades two months in a row, to avoid wash sales.

When a loss is booked, it is used to offset any capital gains in that year. If there is more loss than gains, up to $3000 can be used to offset ordinary income. Any excess is carried over the the next year.

The spreadsheet allows you to specify a start date and then tracks all tax loss harvesting after that date up to December 2025.

When studying the spreadsheet's output two things jump out:
1) If a buy is the lowest price that will be ever seen going forward, there is NO loss harvesting ever again.
2) Eventually a buy will be at the lowest price.

Also:
3) If the starting date is just before a BEAR market, a tax loss harvest will happen about every other month.
4) At the end of a bear market, there will be no more tax loss opportunities for a long period of time. Perhaps never again. For example, after the 2001/2002 bear market, there are no more tax loss opportunities other than 2 small ones in the 2008/2009 bear market
5) If the starting date is just before a BULL market, there will very few tax loss opportunities, perhaps none.

Examples, using SPY/SPY as the holding pair:

(BEAR)    Start date: 12/1/2000                             
Trade #   Date      Loss                Year      tax loss
1         Mar-01    -6.02%              2001      -20.69%
2         Jul-01    -1.01%              2002      -18.10%
3         Sep-01    -7.29%              2003      -3.47%
4         Nov-01    -6.37%              2004       0.00%
5         Jul-02    -6.96%              2005       0.00%
6         Sep-02    -8.52%              2006       0.00%
7         Nov-02    -2.62%              2007       0.00%
8         Jan-03     0.57%              2008       0.00%
9         Mar-03    -4.04%              2009      -8.05%
10        Feb-09    -4.33%                        
11        Apr-09    -3.73%                        

(BULL)    Start date: 1/1/2009                              
Trade #   Date      Loss                Year      tax loss
1         Apr-09    -3.73%              2009      -3.73%

(NEITHER) Start date: 2/1/2015                              
Trade #   Date      Loss                Year      tax loss
1         May-15    -0.65%              2015      -8.49%
2         Jul-15    -0.80%              
3         Sep-15    -7.03%              

Link to google spreadsheet: Google: tax loss harvest.xlsx

To change the start date (row) you can copy it to your own account or download it.