It depends on the furnace output and ambient temp. The smaller the heat source output, the lower the temp, the more time/effort needed. Insufficient heat output or too low a temp and you may not get there (eg the output of a light bulb won't get there at 0*F)

Here's a table to convert watts or kilowatt (your electrical rating) to BTU http://www.energy.iastate.edu/renewa...18_apen_a.html