Energy weapons have a shit load of capacitors that discharge when you use the energy weapons. In fact the RAIL gun could be categorized as an energy weapon excepting the fact that it shoots solid mass objects instead of pure energy. So in other words just like our current energy weapons we have, we are only going to be able to use the tractor beam with a certain amount of time before we would need to recharge it. With that said the duration in which you can the tractor beam is going to be totally dependent on how big of an energy core/source you use. This is also probably one or a couple reasons why RSI has not released the tractor beam in the game thus far.
Oh and they probably will also include a safety switch or something along the lines of G-Safe to make sure, while the switch is on, that you do not tow anything over the max mass limit.
http://www.space.com/13467-tractor-beams-work-nasa-studying-3-methods.html