Human leptospirosis takes a while to incubate, and the normal range between exposure and illness is 3 to 14 days, although it can take up to 21 days. It’s considered extremely unlikely that the illness would show earlier than 24 hours after exposure, even if the patient was otherwise unwell. In rare cases the incubation time can be very long (several weeks) but we normally assume that if there is no illness after 30 days then infection is either not present, or was subclinical.
Illness that develops within 12 hours of the exposure event would not be leptospiral in origin. Often infections that involve contaminated water can show illness very rapidly, caused by the effects of other unrelated bacteria and viruses in the water (such as E.coli or cryptosporidium) or from some chemicals, and while these would not in themselves normally be life-threatening, they can mask the later symptoms of leptospirosis.
The incubation time depends on the strain of bacteria involved, as some strains reproduce faster in human blood than others, but the main factor is the size of the ‘inoculum’ – the dose of bacteria that entered the patient during their exposure. Although it’s perfectly possible to be infected from a single bacterium, in reality the illness develops because the rate that the bacteria are reproducing is faster than the patient’s immune system can control. Bacteria grow by splitting in half, so one becomes two, two become four, and so on. If the patient received a large number of bacteria from the initial contact then the numbers in their bloodstream will be larger, and increase faster – hence the illness develops sooner.
It’s very difficult to predict the incubation time in a patient, but in very general terms the concentration of bacteria in the inoculum will be important (water from a large clean river will have many times less bacteria per liter than urine direct from a rat) and the volume that enters the body (infection via small cuts to the skin usually involve very small volumes of liquid, but swallowing water after a fall into a lake will of course involve far more. The balance of course is that the situations where patients suffer a high-volume intake are usually those where the liquid has a low concentration (you are unlikely to fall into a tank of rat urine).