Yep, the beta is really just like any other version. It has a 30-day evaluation period and if you have a license key, you can register it move it into licensed mode.
Some beta products do have a specific lifespan, and will automatically stop working past a certain date. This product will not. Technically, you can use it for as long as you like. But as Magic_fubu said, it's going to have some bugs, so I wouldn't recommend it. Once the final 2.6 version is released, it would be advisable to move up to it.
There are plenty of ways to add advertising. You can use banner sets to rotate between simple image file ads, or insert ads directly into the layout, or use the screen sets to insert full page ads.
You'll have to get a little familiar with the Layout section, but there are plenty of people here, myself included, that can help.