The size of the game does make a difference. So does the spec of your machine. So the question cannot really be answered.

If you have a small game, with few assets, it should be pretty quick, though ;-)

But if you have a small game and a terribly slow machine, it won't be that quick. And if you have a moderately sized game and a very fast machine, it's still be pretty quick.

One really nice thing is that you don't have to export to the iPhone all the time for testing as you can already do a lot of play-testing right inside Unity. So, in Unity the tweak-build-test-tweak-more-cycle usually is "hit play, test, tweak" which is much quicker. Then, when you're happy with the results, you do a build (which usually doesn't take terribly long) and do final "on-device-testing" (which obviously still is a bit different with mobile devices than testing Web players / standalones which run almost identically in the editor as the do in the final build).