Depends on lots of things. In the old days of 35mm film, and my early digital DSLR days, I used to use the old 1/fl length rule, i.e. 100mm focal length lens would require at least 1/100th second. Then I got older and probably shakier, plus I upgraded to a 24MP crop sensor, capable of detecting smaller movements, and I found I needed to double that for good sharp results, i.e. 1.200th sec for 100mm focal length. Then I started getting clever about keeping very steady, such as by leaning against walls or trees, and discovered that with great care I could sometimes manage to get a handheld really sharp shot with 100mm focal length at 1/3rd of a second -- that required firing the shutter in between my heartbeats.