The last bridge job I welded on was about 9-10 years ago. It was a repair job on a hit bridge on I-380. Every weld on the main beams where x-rayed before the state inspector would sign off on the job being complete. I used some 7018 rod on that job that where years old but had been stored inside.
My main point being that if you store the rods with low heat they will stay dry and last a long time.