A common question at the counter of a battery shop is "Can I leave this maintainer on the battery all the time?" 

Short answer is that it depends on the maintainer.  Most all Automatic maintainers are designed to be connected to a battery and in theory you could walk away forever!  While the battery wont last forever on a battery maintainer as the activate material inside the battery will eventually break down and cause a failure.   You will see a dramatic increase in the life expectancy of your battery if a maintainer is used! 

One of the most common maintainers on the market today is the "Battery Tender" brand of Automatic Battery Maintainers. 

According to the good folks at the Battery Tender.

"In theory, you can leave the Battery Tender® Plus battery charger connected to a battery forever. That’s a really long time. Sales people like to say, “Just plug it in and forget about it!” However, practically speaking, it is a good idea to check on the battery at least once every couple of weeks. Strange things can happen....." - Battery Tender FAQ Page

You always want to double check that the battery maintainer you're purchasing has an automatic setting that will read the batteries voltage continually, and change the battery when it falls below an optimal state of charge. 

In the Dudes experience, we've seen battery maintainers double and even triple the life of small batteries that aren't used year round!   For Example:  Motorcycle, ATV, and Lawn & Garden batteries!