Fermentation has lasted 15 days and still going - is that loooong?

I'm not worried or anything just wondering if a 15 day fermentation is at all crazy? Now

I'm not taking measurements I just see the airlock is still bubbling, the difference from this batch and others is that, I have always used Notty and Safale dry yeast but used WL california liquid yeast with a starter for this one.

With the dry yeast my airlock stops bubbling around 4 days later usually.

not crazy at all, if your temperature is slightly below optimal for the yeast your using the fermentation can stretch out, the yeast taking longer to reproduce. so fermentation will continure longer. I cant gaurantee this is what caused a long active fermentation. but its a possible cause.

Your on the right track, I don't think my basement is too cold but it is cooler. It stays around 62-65 (air temp) so fermentation could be just under 70 which I thought was optimal but that could easily be it.

I fermented on the high side all summer so I can't wait to taste these beers with lower ferm temps.

The airlock activity can also be carbon dioxide coming out of solution. Airlock activity is a very poor sign of fermentation activity which is why spending $40 for a Chinese refractometer or hydrometer is well worth it when it comes to determining when a beer is done. Plus, it is nice to know what the OG, FG, and ABV are. I'd let it sit for 3 weeks which should allow the yeast enough time to clean up their waste and settle out of the beer.