Anything around 20 degC ambient temperature should be fine. More important is that the phone can easily dissipate heat. It should not be covered by blanket or near a heater or in direct sunlight.
I guess you've missed my point, which is: I don't work for my phone. My phone works for me. As such, I shouldn't find myself in the situations where I have to create the optimal conditions for my phone, but rather my phone should somehow accommodate the conditions that I deem optimal for me. The idea that somehow I should provide "cool temperatures" in order to "give a much better lifespan" to my phone while it is "fast charging" ... seems completely idiotic to me. Cooling my room to 20C all year round will incur huge electricity costs, but even if the cost was zero, I don't work for my phone. The other options -- whether slow charge, or replacing the battery, or replacing the phone -- render the option of cooling my home inconvenient to the extreme.
