That's not really a question that has an answer. It depends completely on the size of each individual .cia, and the read/write speed of your SD card. On a slower SD card (class 4 in this example), games such as Pokemon Omega Ruby can take somewhere around 30 minutes or more to install. Smaller .cias, such as emulators or virtual console titles, install almost instantaneously.
As far as the microSD card adapter question goes, I haven't tried it so I can't give you a "it works; I tried it" answer, however I don't see why it wouldn't work just based on how I know those adapters function.