Reliable keyword density checker

When a website is punished for keyword stuffing, does it mean that this website is can be dropped out of the SE’s for these keywords?
yes

After decreasing density a website will return back in the SE’s for these keywords? How long would this take?
Yes, after reducing the density, when the bots crawl your site again, they’ll take note of it and put you back.

As to how long it will take to regain your ranking….No definite figure, but i’ve heard of people who took roughly 1month+, and people who never regained their rankings

WEB CEO freeware has that feature.

I use WEB CEO and the tools seem to work really well. They have a free version.

Does anyone know the current keyword density percentage? and is it affected by the size of the site?

for example. It used to be simply best at about 3.5% on each page. If you use “online casino” that often these days it seems to be a bad thing…

I use http://www.webconfs.com/keyword-density-checker.php

Seems good to me