Is there any useful strategy for when to take defender bonus?

Solution 1:

You are eligible to claim the "Defender Bonus":

• Once every 21 hours
• If you have at least 1 Pokémon in a gym at the time.

One you have claimed your bonus, you will need to wait 21 hours before claiming it again.

You can gain additional bonuses if you have 1 Pokémon in other gyms as well (you can assign 1 Pokémon to multiple gyms, but you can't have multiple Pokémon in the same gym), which will increase your reward. However, this still is only available once every 21 hours, from the time you last collected your "Defender Bonus".

• Adding Pokémon to new gyms does not reset the clock
• Losing a Gym does not reset the clock
• Changing the time on your phone does not reset the clock^{[citation needed]}, as all user information is stored on the servers.

Therefore, the only "strategy" I can suggest is to

1. Maintain your gym, so you can maintain your defender bonus
2. After the countdown ends, add Pokémon to as many gyms as you can, then claim your bonus, before you lose the gym.

This has changed, as of June 22, 2017

The Old Defender bonus was replaced with an automatic reward system, granting a player 1 coin/10 minutes a Pokémon was active in a gym (raids will place a gym "on hold", and Pokémon will not claim coins in this period, or lose health over time); to a maximum of 50 coins per day, that resets at midnight, and is automatically claimed when Pokémon are kicked out a gym.

This means that if you have 1 Pokémon in a gym for 8 hours and 20 minutes (500 minutes) you will gain the max 50 coins. Any other Pokémon that are in gyms in the same 24-hr period that you claimed this reward will no longer reward you with coins.

Solution 2:

You have the 20 or 21 hrs after you click the first bonus that starts the clock. You can collect the bonuses for the gyms you have when the clock runs out. Planning ahead of time gives you the advantage of claiming the bonus at a time where it may be easier to plant your Pokemon on several locations. So my tip would be to set it up so that you can claim however many gyms you can without interruption (I like 3AM-5Am in my area). This is especially useful if you live near heavily visited gyms.

Solution 3:

You'll want to claim it every day. My best advice for strategy is not to be too greedy - try to get into a few gyms within a short timeframe and then claim immediately afterwards. This may vary depending on your area, though. A lot of the gyms near me switch between teams every two hours.