@user3715778's answer is correct. Throwing will not make much of a difference in the orbit and so it won't dramatically decrease the time it takes to reenter Earth's atmosphere.
However, At 400 km altitude everything could be considered as deorbiting! Drag limits the lifetime of many spacecraft there, and the ISS regularly boosts itself to even maintain it's altitude. The rate is quite variable and activity of the Sun can heat the upper atmosphere and increase decay rate a lot.
See all the good answers to How long does trash jettisoned by hand from the ISS fall before burning up on reentry? for example. Basically, if you keep something on the ISS it will not de-orbit, at least until the ISS's end of life. However, if you toss it "overboard" it will find it's way to the Earth fairly quickly, and so this is done.
above: from the question Was this large pieces of “space junk” just released from the ISS in the “nadir and retrograde” direction?
According to Space.com's 02-Feb-2018 article Cosmonauts Break Russian Spacewalk Record During Space Station Antenna Repair:
The cosmonauts spent the day replacing an electronics box for a high-gain communications antenna outside the Zvezda service module. Instead of holding on to the outdated piece of equipment, the cosmonauts tossed the original electronics box overboard, dooming it to burn up in Earth's atmosphere.
This answer shows a plot for a generic satellite, though each size and shape will be different. At 400 km the reentry time ranges from a few years to a few months depending on spacecraft shape and orientation (see this answer for example) and solar activity. More about drag in this asnwer and links within.
This answer links to a site called lizzard-tail.com where you can try a calculation yourself. It won't be accurate, again because spacecraft shape and solar activity can vary, but you can play with some general spacecraft and solar parameters there to get an idea. The website also links to this documentation
above: borrowed from this answer.
above: example of ISS altitude vs time, borrowed from this answer.