G'day Col,
Different aircraft have different limitations. I can't think of one right now which requires you to be below a certain speed in ordr to use. The speedbrake is there so if you reach the high speed limit(Mmo) you can pop them out and slow down............hence "speed brakes", they also help you increase the drag in the L/D equation thus increasing the descent profile(particularly useful with say a depressurisation or uncontained cabin fire). Some aircraft require they not be deployed when flaps are set greater than ten degrees and many operators do not allow them to be used below 1000 ft(as standard operating procedure rather than an airframe limitation).
Hope this helps