The nominal value is 1.25uF. The nominal extension is 22.5Hz.
I usually say that it's reasonable to try from half to twice that value. Larger values will give more extension but less power available at very low frequencies - reasonable for very high efficiency speakers or most headphones, or music without a lot of deep bass. A smaller value will give less extension but more bass power capability.
There is substantial interaction with the speaker's impedance, so experimentation is in order, in your own system with your own music. I usually also recommend using inexpensive caps in several values to determine which gives the best in your system, before spending any big bucks on fancy caps.
