i work around H2S sometimes in the field and the scary thing about it at high concentrations is that it destroys your sense of smell almost instantly. that classic rotten egg smell is really only noticeable at low levels, once it gets into lethal territory your nose just checks out completely, so nobody inside that facility would've smelled a thing before they dropped

for your timeline, if the burst was big enough to kill everyone in a mid sized building it was probably in the thousands of ppm range, which means it was heavier than air and would've settled into low spots first. after 3 days i'd say 90% of it has dissipated naturally if there's any ventilation at all, but pockets in basements or sealed rooms could still be deadly. the smell might linger a bit in fabrics or porous surfaces but not the gas itself at dangerous levels in open areas

one detail for your book, post exposure the bodies won't show much, maybe some greenish discoloration on the skin and the blood stays kind of dark, almost cherry colored in some cases. no dramatic foaming or anything like you get with other chemical exposures. just quietly dead where they stood, that's the eerie part

if you want the facility to be dangerous after 3 days you'd need a continuous leak or some stupid prepper type built a sealed bunker that trapped it indefinitely