It takes approximately a century to train and build a company of Space Marines. That being said, a Chapter reduced to 100 men would likely be disbanded, as that's ultimately not even a Company-- if the Chapter were able to recover losses after being reduced that far, it would have to have a relatively intact apothecarion, reclusiam, and logistical support staff (i.e. the Chapter Master's household, Adeptus Astra Telepathica representatives, etc.). More practically, most of the Chapter's hardware would be destroyed, leaving very little in actual non-personnel assets available. Without enough functional suits of power armour, Rhino chassis, and even munitions in general to go around, the Chapter has no way to continue fighting.
For what it's worth "disbanded" doesn't necessarily mean destroyed-- Chapters are often re-founded after disbandment, using the few remaining brethren as instructors and advisors. Re-founding a Chapter requires the attention of the Adeptus Mechanicus Biologis and the Administratum to develop approximately five-hundred sets of progenoids from stored gene-seed tithes. When the Scythes of the Emperor, Lamenters, and other noteworthy Chapters that are severely under-strength finally give way, the most likely outcome is that they will be re-founded. That is the whole point of the gene-seed tithe, after all.
It is possible to remove the second progenoid after five years, but the first is not removable without killing the Space Marine. Both are needed for a new initiate. Removing one just makes it less work when it comes time to remove the other, since it's often under enemy fire that the primary one is extracted.
