I am in CA, too. I had not done observations for a very long time, but have definitely been reminded lately (just did one yesterday) how valuable they are. Districts are very good at spinning the truth to minimize their risk and obligations to provide FAPE, which often equals dollar signs. So, while it may be annoying for you to attend, I learned that observing at the same time as the district/administrator, you can have a more intelligent conversation at an IEP meeting about what was observed. If your BCBA is willing to attend an IEP meeting to discuss their observation, great, but if not, you may want to prioritize being there to offer a likely different perspective that the district will share at a meeting.
For limiting it to 20 minutes, I would push back if you want to see how they are doing in a specific subject, time of day, etc. I often put in writing why it's important to request additional time, etc., and connect it to meaningful parent participation and parent rights to ensure the IEP is being implemented with fidelity. I would definitely require them to put their policy in writing and consider if they keep pushing back if they are required to respond to the denial in the form of PWN.
I am actually going back and forth with the district because they have denied (verbally) my request to be present at their observations. They are building a case, so they do not want parents to see what is actually taking place when they state he is doing everything independently, etc. I think they are aware that if we observe in a collaborative manner, it will make their narrative much more difficult to "prove". So, I am still requesting that they put their policy in writing because two years ago, the policy was the complete opposite, and one of their district staff was required to be at any parent observation.
All that said, keep pushing back and ask your BCBA what is necessary as far as time, as well. Documenting the BCBA's professional opinion and submitting your requests in writing is crucial and is important if you ever file a state complaint or file for due process/mediation. This is speaking as a parent who has participated in both, and what "saved the day" was that we put everything in writing. If it isn't in writing, it didn't happen!