The basic Alternate Form power works like a switch. You turn it on, you have it. You turn it off, you don't. Billy Batson (hero name Shazam or Captain Marvel, depending on who you ask) has a great example of an Alternate Form power. He turns on his super-form, and it lasts as long as he needs it to, or until he gets knocked out.
There are other forms of this power, however. You can create Alternate Form powers limited to a specific condition, such as an elemental morpher who can turn into water when he's touching a natural body of water. In this case, every power inside the Alternate Form has its cost reduced, and when you leave the water, the power turns off.
You can also limit it to a specific number of turns with the Fades modifier, which can result in either an alternate form that gets weaker the longer it's on or one that only lasts five turns, but notice that both of these variations give points back, applying a -1/rank modifier on every ability in the power.
Mechanically, Sean Duggan is absolutely right, you need to spend a Free Action to maintain the effect and it can be nullified, but unless your GM requires you to call out every Free Action every turn, in gameplay it's easiest to assume it's around until you get knocked out or turn it off. But, on the off chance someone wanted to make a character with a limitation on their Alternate Form power, I wanted to put out a few ways to do so.