How long does ant poison stay effective?

It depends on what you use, formulation & application site. Indoors it's lifespan is considerably extended. Outdoors, you are lucky to get a month or so unless it rains which will dilute it out. heat & UV break it down also. For example, on concrete in direct sun, you would probably get the least viable no. of days. Foot traffic (if any) would further reduce it.