Any reduction is good, as sound pressure levels can get very high on a motorcycle (especially at speed

). I would say find something comfortable to wear over the dB reduction, as it is not good having great ear plugs sitting in your pocket. Decibel's work on a scale, so -3dB is half the SPL, -6dB is a quarter. SPL on a bike at speed is high, lets say 100dB+, so a reduction of 30+dB would bring it down to a quitet, managable level.
Also, it is worth noting that too quiet is no good either as you need to hear horns, sirens, pedestrians, ice cream van... Keep your wits about you. I have found the home made custom ones ok, but they dry out a bit over time so don't stay true. The EARZ foam ones on eBay are good for me, 100 pairs for £16. I use them for a couple of weeks then change them. FYI they are -37dB. Hope this helps.