They went from Hunter Gatherers to the a space faring society over the course of the episode. The planet was said to rotate 58 times per minute. So almost 229 years went by per day they were there. The episode never specifically states how long Voyager is there, but I would guess it would have to be at least a week.
It's conceivable that with the drive to see what's up there, and with no major technological setbacks(like say the fall of the Roman Empire in our own history), they could've accomplished the task in 1600 years, which is roughly what a week of Voyager being there would be. That's given a very generous stretch, but there's nothing saying Voyager wasn't there two weeks or even a month really.
It's an episode that makes less and less sense the more you think about it really. I just take it at surface value. An average episode with an amusing premise. Michael Taylor usually comes out with more good episodes than bad, and I'd say this one falls in the middle of the range. I find it difficult to credit that they supposedly didn't realize this was an extremely similar story to Wink of an Eye, from everything from aliens existing in faster times to the title, until they were already in production.
The implications about how technologically advanced these aliens should be years later is startling. They should either be the more advanced power in the galaxy by the time Voyager gets home... unless they wiped themselves out. Just think if Voyager hung around another week, they might've had a chance to acquire some better transwarp technology or something. Or really gotten blown up.