The 24" griddle on our RNB will technically hit 325°f in about 7-10 minutes. HOWEVER, the surface is quite uneven at that point. Depending on what you're doing you should give it another 10-20 minutes for as much of the steel to heat up as possible.
When we turn on one side to use it as a linear french top it hits 200°f in about 4 minutes.
More: https://bamasotan.us/griddle-faq/
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Nice La Cornue CornuFe.
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