Why do the girls bother calling and asking when the seatbelt sign is going to be turned off??
How about so they can plan their service? If they're in the middle of a meal service and they know the belts will be on for a long time, then they'll go ahead and clear the trays without offering tea or coffee. Then again, if they're likely to be turned off in the next few minutes they might leave the trays out for a bit longer and do the tea/coffee once the signs go off.
They don't know what's causing the turbulence, so how about telling them? If you don't know how long the belts will be on, then say so.