The bear has a limit as part of the game balance. It's a non-mana summon so it lends itself to be overpowered. Early game, it's quite powerful. Mid-game, it holds its's ground. End-game, it's a throw away summon for when you're running away from things. There was quite a discussion in the beta on what time limit made the most sense, a lot of differing opinions on that point.
Keep in mind you get the damage boost at all times.