redis memory

I currently set the redis memory 256M.

Q1.
Is there a way to see this memory is enough or not? I can see the metrics but it somehow shows moderate graph rather mild and it’s like normal mild wave.. even I change the memory to 64M, the metrics doesnt change much… that lead me to next question.

Q2.
Redis has a mechanism/logic whereby it auto flush out the memory to take the latest data? or once it’s full, it will not take the latest data anymore? I turned on the Redis on a site for a month with 6K visitors, the metrics never shows it’s hitting max or something (always moderate..)

Thank you