You should capitalize "Twinkies" and place a comma after "1998".
In AmE, we would say "on the market" or "in the marketplace". I don't know about BrE there.
I would write "prior to 1998"; some would use '98. All three of your endings are grammatical, but there will be disagreement about whether 6 years is a long time or a short time. Since Twinkies were invented in the 1930s, I assume your 1992 (or '92) figure refers to a market outside the US.
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