The hard fork would need to be written (<1 week + 1 week code reviews, could be done in less). The fork needs to be accepted by the shareholders and considering the low difficulty a large percentage should be used otherwise the “majority” might be misleading w.r.t. the actual users.
What then happens is really beyond my expertise. It is very hard to predict what people will do. A sad fact is that most coins (not just NBT) are held on exchanges or online wallets and many people in fact don’t even know how to install a wallet although they deal with crypto tokens for years. That plus teaching them parking, plus the C- class user experience of the NBT client will likely reduce the number of people who actually park their NBT to a low number, but especially holders of large sums will probably do the effort.
Then it is a simple linear function. If you assume 30% of the supply to get parked then there is about 300-400k NBT in circulation. Draining 10% per month would reduce that to roughly 28%, i.e about 100k within a year.