It seems a steady decline, but as it has been pointed out if your lip seal is not good due to bulbar symtpoms the values may be falsely lower. FVC is not the only available test. Respiratory specialist should offer further testing and provide Bipap if required.
Personally, I would want to know if the figures are correct (re-test) and would no wait until FVC gets any lower to seek assistance. Insertion of PEG, for example, can be more complicated (and has even been refused!) due to a low FVC. Also, my understanding is that anything below 30% requires extensive use of Bipap (some people with very low FVC use it 24/7) or a discussion about venting. Joel always writes about the advantages of early venting versus waiting for an emergency to occur.
Having said all this, some people's FVC (judging by figures on Patients Like Me) seem to steadily decline up to a certain point, and then remain stable for longer periods of times.
All the best, Dani