How long does it take for Salmonella to effect ?

My Four year old sister got really sick we thought it was a virus until her symptoms got worse (abdominal pains, diarrhea, and, dehydration) so my parents took her to the hospital when they came back they gave her a prescription . The medicine didn't really help . until today my mom got a call from the hospital saying that she got salmonella . So were wondering where did she get sick from school or home because she got sick on the last day of school June.2nd.2011 and She has not been eating any kind of meat at home and at school the lunch isn't to good . so how long does it take for salmonella to effect?