You'll want to use flashcards during "trapped time", i.e., time in which you can't otherwise efficiently gain meaningful exposure to your target language, and minimize their use at other times.
To take my own situation as an example of how to make use of trapped time, pretty much every day I'll use my SRS on my commutes to and from work, from door to desk, and I'll sometimes use it when wandering around my office. I'm also apparently among the 40% of people that use their smartphones in the bathroom. Outside of those everyday occurrences, I'll also use it when I end up standing in a line, on hold on the phone, and generally whenever in transit from point A to point B. Only very rarely will I use SRS outside of those times. The sum result of this is that I typically spend about an hour each weekday using my SRS. (My SRS use on the weekend is typically very limited because my work commute is removed from the picture and any trapped time I'd otherwise get will be with a gaggle of kids that'll typically prevent me from using my SRS.)
I think Randy would recommend that I spend that kind of time using my smartphone to listen to music, listen to podcasts, read a book, or watch a movie. Indeed, I sometimes do listen to music or podcasts while using my SRS. However, the problem with all of those is that if you do not already know the words that are being thrown at you, you're getting exposure but it's not meaningful exposure. You can eventually learn sufficiently common words solely through exposure, but doing so is inefficient for the same reason it's inefficient to learn grammar solely through exposure; you'll have to go through a number of completely meaningless exposures before the meaning of the word finally clicks for you (more on that later). That is, if it ever clicks for you; you'll get enough exposure for that to work for the most common words, but at a certain point you'll see the words too infrequently for that to continue to work (more on that later).
If you don't follow the "exposure only" dogma strictly and you look up the words as you come across them, then you can make exposure to music, podcasts, books, movies, etc., meaningful. Although that's my typical approach, it's a hassle when in the situations described above. For music, podcasts, and videos, you'll need to identify a word you don't know. If you can catch it in the audio file, great. If not, then you need to try to find lyrics or a transcript. Once you get the word, you'll need to look it up. Typing on a smartphone is time-consuming while walking around. Even if you can copy and paste the word, if your dictionary is online, you might not have access to it at all (e.g., you're underground on a subway). Even when reading, you'll need to stop and copy and paste to a dictionary, and I haven't found a convenient way to use Learning with Texts yet while walking around with a smartphone in hand. In short, it's a huge pain and that pain means fewer meaningful exposures per minute.
With flashcards, on the other hand, you can get a meaningful exposure to a word you don't know every several seconds without fail. Flashcards then take time that would otherwise be dead for language learning—or, at best, very inefficient—and allow you to get rapid exposures to what you're learning.
Your ideal situation then is to strike a balance: you generate just enough vocabulary that your SRS reviews fill up your trapped time completely. If you have too much vocab to finish during your trapped time, then you'll want to spend some of your otherwise free time reviewing flashcards, but you'll also want to keep this to a minimum (while still getting through your reviews) so you can focus that free time on language pursuits that are more interesting to you. If, on the other hand, you don't generate enough vocabulary to fill up your trapped time, you'll either need to generate vocabulary more quickly or find more time to generate vocabulary to make the most use of that trapped time.
This post is part of a series on using flashcards written in response to Why I don't use flashcards (and you shouldn't either) and other related posts on Yearlyglot.com.