In economy, I’ll use the bell with discretion and infrequently, e.g. not in the middle of a service run and not be demanding. EK and EY seem to prefer you to call than turn up in the galley and are pretty good at answering.
In business, I’ll use it as often as I need to, since the fare is typically 4-5 times more and I expect the service. If the bell isn’t answered in business (happened this morning on TK, missed 3 times and answered 10 mins later on the 4th attempt, I will ask to speak to the senior cabin crew member and ask why – it usually means it is answered quickly thereafter.
As cabin crew say, when the bell goes off, it could be a ‘coke or a stroke’ (or a bad nosebleed) – it should be answered within a minute or less.