Hello,
The Arcadia D3 12% flourescent tube bulb is a great bulb, slightly better than the Reptisun 10% flourescent tube bulb.
I always recommend replacing flourescent tube bulbs every 4-6 months, just to ensure that their output stays at higher quality. You can always continue to use it, just for extra brightness in the tank if you wish to, but I would replace it.
It is an upgrade from the 5% light, which was not quite as strong, but lasted roughly 6 or so months with good output. Here is a UVB chart to show you the gradual decline in UVB emissions that the 10% had:
http://www.uvguide.co.uk/fluorescenttuberesults.htm
I hope that this helps you out in determining the factor of decay of the bulb. If you have any questions, please ask.
Tracie