I was recently diagnosed with a medical condition that requires movement every 45 minutes. I travel a lot and find that frequently seat belt signs are on for well over 45 minutes. Had a flight from Nashville to ORD earlier this year that due to weather the sign never came off. Opinions from experienced flight crew as to the probability seat restrictions will occur for more than 45 minutes?
If you're on any American airline, seatbelts seem to be on almost continuously and the result is that they are completely disregarded. On other airlines the seatbelt sign is turned on during takeoff, turbulence and landing and turned off otherwise, and is respected while it is on.
Typically on a US airline such as American, the aircraft takes off and the flight attendants are released from their seats at level flight. At this point, or earlier if anyone chooses to, and the aircraft is not showing signs of turbulence, passengers will remove seat belts, get up, retrieve items from their luggage, and use the bathrooms. The flight attendants will show no concern about this as it is completely normal.
The seatbelt sign will typically stay on. Sometimes this lasts for the whole flight even if it is 5 hours from New York to San Francisco, but sometimes it is turned off following an announcement from the pilot that the weather is particularly stable. During the flight, even if the seatbelt sign is on, if there is a particular turbulence, no matter how weak, an announcement will be made that you should return to your seat.
Only if the flight attendants have been ordered to remain seated themselves, will there be any objection shown to passengers getting out of their seats.
I'm not flight crew, but fly 100k+ miles per year and I'm sure you won't have any problem in the US. I've been sat next to people who don't even bother to put their seatbelt on at all, and flight crew haven't bothered them.
I would not take off my seatbelt on BA while the seatbelt sign is on, but on any American airline I do not hesitate and have never been challenged. I suspect corporate policies that are intended to prevent the airline being sued, are responsible for this. Personally, I think it's going to backfire at some point. All my own opinion.