1: depends on your upstream system.
Can you get liquid carry-over from an upstream drum, or is it condensation only. If you can get liquid from an upstream vessel you would take x seconds of liquid-full flow. (x depends on safeguarding) The resulting liquid volume should fit in your vessel.
2: depends on your downstream system.
Why do you have the drum: corrosion reduction, or will a slug of liquid cause an 'explosion' when expanding in a hot vessel downstream??
In other words, have a look at your situation and see what can happen upstream, and what do you want to avoid downstream...