How does Duolingo decide when a lesson needs refreshing?

[StephenInd]

I am finding that refreshing lessons that I have completed is taking too much time. I hope that by understanding how Duolingo decides when a lesson needs to be refreshed I will be able to meet its performance requirements and so have to refresh lessons less often.

Really appreciate any help. Thanks.

[pentaan]

How does Duolingo decide when a lesson needs refreshing?

http://duolingo.wikia.com/wiki/Decay#Decay

...... and so have to refresh lessons less often

My favourite method
https://www.duolingo.com/comment/6176795/Best-Way-to-Make-Your-Tree-Turn-Gold-and-Stay-Gold

Note:
You don't have to refresh your lessons. Refresh them only, when you feel that you need it!