There’s different species with different lifecycles and behaviour. We have the ones that use thermal imaging to jump off of trees onto unsuspecting heat sources. In other areas they hide on grasses and climb onto any animal that brushes past. These ones tend to stick around longer IIRC.

The ones that drop from trees usually take around half an hour to migrate to a feeding location; then once they drill in, you’ve got about 2 hours before they become a health hazard by vomiting some of their own fluids into the wound. Before that stage, they’re just a nuisance.