We have the term "Greatest things since sliced bread!" but how great was sliced bread? Was it actually considered a huge deal when it first hit the market?
I cannot say how quickly it spread and whether people at the time considered it to be a big deal, but it did make the newspaper in the first town where the first commercial slicer operated (Chillicothe, MO) in 1928 (that is, the first time bread was sold pre-sliced. People had been slicing full loaves for some time before this.) The impact of the slicer was heralded at the time by the marketing folks behind it as “greatest forward step in the baking industry since bread was wrapped.”
Wonder Bread, which was already the leading bread brand by the early 1930s, was the first national brand to come pre-sliced. They also heavily used a marketing campaign which called for people to "wonder" at the new style of bread, most likely adding fuel to the fire that sliced bread was something extraordinary.
By way of speculation, I would suggest that most people, even at the time, used the phrase in an almost tongue-in-cheek fashion, much as many people do today (although it's become such a basic idiom nowadays, that the tongue-in-cheek nature might have worn off).
http://chillicothe.newspaperarchive.com/chillicothe-constitution-tribune/1928-07-06 (you have to register, but you can use throw away info)
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