Question: When should I hit my forehand flat?
When you hit flat, none of the energy of your swing goes into creating spin, so all of it goes into driving the ball forward. You get the most raw power when you hit flat, but without topspin to push the ball down into the court, your margin over the net on hard, flat shots is small, making them risky. Compensating somewhat for that risk, though, is the greater simplicity of executing a flat shot: the longer alignment between your string bed and the ball makes your contact more reliable. This certainty of contact makes less aggressive flat shots, typically with a short swing, often a good choice in defensive situations where you can't set up properly to execute the more difficult timing of a spin.
Most players can hit flat forehands from a little below their knees to a foot or so above their head, although the more Western your grip, the less comfortable flat shots become. Truly flat forehands are fairly rare at the advanced level, but compared to the immense topspin some players use on their forehands, a slight topspin looks pretty flat. Many so-called "flat hitters" are only relatively so.
Index of 9 Forehand Questions