Can a person go to jail for a couple days for beating someone's brains out?

You go to jail for a few years.

Actually you go to prison for years in that case   

it is assault and battery

No. In such a case, it would be prison for a few years.

Is the victim now dead or alive??  

That is, at minimum, aggravated assault. In fact, it may even be attempted murder, depending on the victims condition, if you are trained in martial arts (martial arts training automatically qualifies as a deadly weapon), etc. You are looking at a couple of years, not a couple of days!

You will likely go for many years; could be 10 to 40 years, depending on injuries. 

You go to pris for a few years. You have to prove they threatened you, and your only recourse was to fight back. You can't beat them because they made you cry.

oddly enough, in both Los Angeles and San Francisco, a person who seriously injures a mugger or attacker in self defense if far more likely to face criminal charges than a homeless person, illegal alien or gang member with a criminal record is for committing assault during a robbery....

that's because LA's new "progressive" DA is from San Francisco and is trying to push through all his "progressive reforms" here....

one of the first things he pushed was a new law similar to one that already exists in SF that would make a driver criminally liable for any damages that result from resisting a carjacking...

that's right...if a guy smashes a woman's passenger side window, brandishes a knife and yells "get the F- out of the car" on a deserted street with her baby in a rear child seat....causing her to panic and accelerate, SHE is now the criminal if she runs over the guy's foot while she flees...

yay progress....

That's felony assault, so you might go to jail immediately.  If so, you may stay there if you are still considered to be a danger.  Further, if and when it goes to court, you may find yourself in prison for a while.