~5 gallon rum barrel questions

I was lucky enough to procure a small barrel that was used to age rum from a local distillery. I want to age a porter in the barrel. The barrel is currently empty and sealed with a rubber stopper.

How long can I leave it empty?

Will it last 10-14 days for a beer that I can brew tomorrow?

Should I fill it with something else that I already have on hand?

Do I need to do anything else to "condition" or "treat" the barrel?

Thanks!

I haven't done this yet but I have read up on it a bit. If it is completely empty then you should throw some rum into it to keep the wood wet, otherwise you should be good. The alcohol will be enough to sanitize it.