Just how many time travel stories have there been in Star Trek. And how many equal too many or "dependent". Even ENT, which included time travel as part of its design didn't do straight up time travel that often.
I did some number crunching awhile back for a post in the Trek XI forum. You can read the original post here. My primary source was Ex Astris Scientia's time travel page.
TOS had 5 time travel episodes out of 79, or one for every 15.8 episodes. DS9 had the fewest relative to the total number of episodes (9 out of 173, or one for every 19.2 episodes). TNG had 12 out of 176 episodes, or one for every 14.6 episodes.
VOY had 13 out of 168, or one for every 12.9 episodes. ENT was the worst, with more than one episode in ten being a time travel episode (10 out of 97, one for every 9.7 episodes). That's not counting ones where the TCW was only mentioned in dialogue. Excepting the anomalous DS9, there's a consistent increase in the percentage of time travel episodes from series to series.
When I mentioned to a non-Trekkie friend that Trek XI would contain Trek's #1 most overused cliche, time travel was his first guess.
In the post I linked to above I quoted the opening lines of the filksong "Let's Do The Time Plot Again", by Tom Smith. The most telling line is "What was done uniquely/now is practically weekly". It doesn't matter that some time travel eps are good, or that time travel in abstract is not a bad plot device. The problem is overuse, not what they overused.
Marian