A standard by which US presidents are judged is by what they achieve in their first 100 days. The benchmark is the first hundred days of FDRs presidency where he began the New Deal.
I believe I once overheard Piers Coleman say that he thought new postdocs should produce a paper in their first 100 days in order to gain momentum and to position themselves for their next job.
This seemed a little extreme to me and almost impossible for many projects. But perhaps that is the point! You should not embark on ambitious projects (especially developing complex new software and building new equipment).
I aim for my postdocs to have submitted a first author paper within 6 months of starting. Most have and I think this has helped build confidence and momentum.