Is the heater limited by power or temperature?
If it is power then you just balance the input power (to the bar, this may be limited by the thermal conductivity of the material) with the convective and radiant heat loss from the bar.
How long it will take is another problem altogether.

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P.E. Metallurgy, consulting work welcomed

144x: Look at two things - how the heat gets added to the bar, and how the heat gets dissipated from the bar. The addition portion is a result of the heating element - convection (indirect contact), conduction (direct contact), or both. Both have limits on how the heat gets to the bar (thermal efficiency). Likewise, look at how the heat gets out - again, convection and/or conduction. There is an efficiency to this part of the process as well. How the two sides of the equation work together determines how much heat the bar can contain - and thus it's final "steady state" temperature as a result of a given thermal input. (Note: this also applies to COOLING a bar!)

Converting energy to motion for more than half a century

Is there an example or guide or standard that has the formula that you can guide me to?

Any basic college text on heat transfer will have a similar example.
Find an ME and borrow one.

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P.E. Metallurgy, consulting work welcomed

Omega Engineering, Chromalox, and Watlow have good handbooks/catalogs for calculating heat losses empirically,