How long in duration .... about 1/2 hour

how often .... depends on the brand/quality of strings .... I use Elixirs and with these about every 2 months with daily playing.

When I change the strings (yes, one at a time) I initially tune them to a higher pitch. Leave the guitar in the case one day, then tune to desired tuning, Very rarely do I have to re-tune, and when I do it's always the 2 E strings.

biLL