There is no Death Note in real life

The same amount as someone without it. Mine is still on my shelf and has no effect on my life expectancy

Might do better in no stupid questions or in an anime discussion community.

To answer the question, probably quite a while. Kira irl would likely accomplish their goal with almost no resistance, as long as they don't make stupid plays. Magic doesn't exist irl so all known forensic and investigative techniques would fail. Not to mention, if they harness religious fervor they'd have too much support to even try to act against.

That is, of course, assuming Death Note the anime doesn't exist in this hypothetical. If it did, then Kira is fucked six ways to Sunday. Probably wouldn't last a week.

I would say at least as much time as it takes to get an appointment with the real Santa Claus (not with one of those many actors, malls & shops everywhere are using to lie to kids) times the time it would take to tame and then to train the Easter Bunny, persuading it to lay actual eggs instead of chocolate ones.

But it could be done much faster provided one can somehow get Rumpelstiltskin to transform gold into straw. Which is rather difficult.

How long is a piece of string?