The estimated length of a game is something that can often tell you certain things about the genre of a game, at least that convention was true in the past. In the good old times of the Playstation 1 the content length could also be estimated by the amount of disks needed, but that's another story.
Most of the time when there was a discussion about a game's length it referred to the time needed for a full run without loitering and completion of a reasonable amount of sidequests if existing. This estimate of gameplay length was intended as a guide to guess how long they will need themselves. Of course, given the tendency that a lot of games incorporate minigames and other optional possibilities that guesstimate is rather inaccurate. There's nothing wrong with that.
What is wrong is the tendency of artificially prolonging gameplay time via activities that the majority of players won't find enjoyable. Often these activities feel a bit disconnected from the main gameplay and/or the plot. Some of you might argue that this is the way that subscription based games 1 make their money. Artificially prolonging gameplay time may not be a nice tactic, but for this kind of service it's the standard business tactic.
So, what I'm trying to say is that the artificial prolonging of gameplay time is indeed detrimental to a single player game. The more story driven it is the more this rule applies.
If you need proof for yourself you can check out Mass Effect 2 (ME2) and Assassin's Creed: Revelations (ACR). Both of those fall horribly into this trap.
ME2 has the minigame in which you are mining for minerals via launching probes onto various planets (read: pressing one button over and over again) and scanning planets for the minerals (read: moving an infuriatingly slow target reticle with the analog sticks or the mouse). Sure, the minigame isn't necessary to advance the story, but you'll need the upgrades for weapons, etc. in order not to die.
ACR has even two of those atrocious flaws.
The first is called "Den Defense". It's basically a built in tower defense minigame that does nothing in terms of plot nor is it particularly good in terms of equipment or monetary revenue. Add to that the fact that the camera angle is a pain in the arse and you have something you will want to avoid at all costs - since losing one of those minigames has annoying consequences. Furtunately Ubisoft gives you an option to avoid those nearly entirely, making them pointless in the process, as Yathzee pointed out. 2
The second flaw is "Desmond's Journey". Aside from the fact that this must be Ubisoft's try to breath some personality and background into one of the most generic, bland and plain boring characters in recent gaming history the first person puzzle game sequences suck. They are counterintuitive, frustrating and incredibly clunky when compared to the main gameplay. Given that they proved to be almost unanimously hated on the Internet I'm still amazed that Ubi thought it a good idea to release additional content which prominently features the game's worst mechanic - which would be the first person jumping/puzzling in Assassin's Creed Revelations.
Now, I don't say that minigames shouldn't be done. Rather, my point is that the developer should have a clear focus of what makes the game great and how the minigames relate to that as well as how to advertise the distracting content ingame. Be aware of your strengths and don't let the unimportant things take more time away than necessary from the core gameplay experience.
Update: Typo correction in title