oh....that actually might be more epic
Yeah, stopping time would be pretty ridiculous. But seeing as how with a time limit on it, it would really only be slowing time, because time would continue, since there would be an "invisible timer" for your shout that would determine when it shuts off, and time moves forward again. Basically, a stop time shout would be about equivalent to pressing the inventory button to pause the game so you can change equipment or do whatever. So....there's really no need for that in the game, since we can already "stop time" by hitting pause to go have lunch and then come back to finish the fight.
Anyway, sorry if that was unnecessarily confusing. Point being, the benefits of the Slow Time shout will be huge, especially since apparently it gets even longer duration as you upgrade the shout. Since level one of the Slow Time shout has a duration of 8 seconds, we can assume that the next 2 levels may be 16 seconds, then 24 seconds, or perhaps longer, even. With that being known, I'd say that shout will see some serious use at the basic 8 second level, and in really desperate situations, players will use it's full length just to stay alive in a near death situation. Of course, with each higher level of the shout, there will be a drastically increased cooldown time, so the lowest level will probably be used fairly often, with the highest level being used perhaps only once a day.
As for the effect it has, according to the OXM description, it allows you to casually engage multiple enemies akin to a medieval Max Payne. With that in mind, I'm very excited to use it, and I think that should give you a better understanding of how it will really work in-game.