Warm ups once a day can be enough, 15 minutes or more.

Yes. Still do the warm up if you haven't done it for the day despite where you left off on the exercises.

You can still do warm ups when drawing stuff for fun if you haven't done it for the day. Once a day is already good.

I am not sure about how many times you need to do warm ups, but doing it once a day is enough for me to maintain curve and line confidence