How long should a shimano 105 BB last??

Mines about 1500 miles and started creaking.. thought they might last a bit longer

Creaking does not mean the BB is worn out. It needs refitted, or it may be pedal or chainring bolt threads.

I initially thought a pedal but wasnt. I pulled the cranks off and checked the cups and pedals the other night.. didnt think to look at the chain ring bolts though. Drive side cup was a little loose which i tightened. Makes louder creaks with increasing pressure??
By re-fitting do you mean take both cups off and re-fit?

Yes, with grease on threads.

Thanks Al 8)

Until you get to Carlisle!

Mine was creaking the other day. Fair enough, I thought, it has been on there for nearly 5 years with no attention (105 external cups). Took the cranks off and amazed to find bearings still feel really good, and abundance of original green Shimano grease still in evidence. Cleaned, greased threads, reassembled and now perfect. It must have done thousands of miles and I used to commute on it in all weathers – very impressed with it.