HEAROSUltimate Softness Series Foam Earplugs, 20-Pair

right now i buyed this product and how many time it will leasts 10 days 20 days 40 days or more ? (sorry for my english)

why do they have to be replaced so often? do they wear out (become compressed and won't stay in) or because they get dirty?

Would these be suitable to completely drown out the voice of the exceedingly annoying member of staff that sits behind me at work?

What is the noise reduction rating on the package?