You need to determine what is the minimum air velocity that will sustain the weight of the model in horizontal flight, solving V from the lift formula.

Since that flight condition will be possible only at the maximum practical angle of attack (just before stall starts), the max CL needs to be found and used.

Vstall = Square root [ 2 * weight / (wing area * air density * CL max)]

For exact results, you need to know the maximum coefficient of lift for that wing at the speeds it will fly.

Then, you need a more exact weight than the estimated one.

http://en.wikipedia.org/wiki/Lift_(force)