The problem is not 'how much screen time', the problem you describe is her reaction when you tell her 'time to get off'.
First, to answer your question: I feel like we are at a point that we are fairly liberal with screen time. We homeschool and have a lot of time at our discretion. We do have balance, though. Like others have stated, other tasks must be done first AND *attitude* about getting off the device/laptop must be reasonable. Kiddo knows that, aside from a short window of time in the morning when we are both waking up, there's no recreational media time until after 3:30 and all of his tasks are completed satisfactorily.
Understand that there is a chemical brain response going on when your child is playing anything on the screen. Just like TV. Little releases of dopamine are happening constantly when your child is engaged that way. So, when you say "all done", figuratively you are telling your child "I'm about to take that 'happy drug' away from you." and then they crash. Then it's meltdown time, tears, ignoring the parent....
We did away with most of that by having clear consequences for that.
First, put a time limit on his gaming with a timer nearby. I ask him to check that timer every so often. I prompt this with "how much time do you have left?" every so often. I hang around toward the end, once he's at his last 5 minutes or so. I give him a verbal 5 minute warning. (My child has some very real challenges with normal, everyday transitions, so he needs more help than a typical kid his age.) If he's watching videos, I have him choose which ones he's going to watch *before* I let him have that time.
When it's time to leave the computer/device, I am clear: first, I am not the timer. The timer is the timer. If you cannot get off in a reasonable manner, you get no more media time for that day. (So, he has about 2 hours a day, but it's broken up in to chunks, which I will explain a bit later.)Or if it's a big fuss, sorry, no media time tomorrow/next week/whatever. When it was really bad we took a break for a few weeks. He is not *owed* media time and will not die without it. We are very consistent about that consequence.
The other piece of it is that we rarely allow him to play for more than one hour at a time. This is to help his brain chemistry stay regulated. After an hour we pretty much have to turn off the screen and go outside, for a walk, come help with some chores,help making dinner, go read a book, go out with your scooter or bike.... I compare it to nursing an alcoholic drink over an hour or so instead of drinking it quickly. Your body will respond better and more reasonably to a few sips every so often than just guzzling it down all at once. Once you have read up on the physiological effects of screen time, you can understand how this relates to any other 'pleasant' chemical the body ingests or produces. That's the same reason some people over-indulge in yoga or running or their own screen time/gaming... it's a nifty chemical, dopamine. What we have to be aware of is our own addictive behaviors. :)
I know it's a wacky way to look at screen time, but that's my philosophy. My son loves playing online with his dad in the evenings, he isn't just cruising the internet for 'stuff', he's very pointed in what he wants to look up or watch. Some days, we don't have time for media, some days, we have more time. I try not to make it something to argue about, more something we all enjoy in our different ways and we impart our own expectations on how he's expected to handle himself when those transitions come up. It doesn't make things perfect, but it does keep them consistent as our son is still learning how to regulate his own emotions and actions when he's disappointed. I try less to 'rule' and more to guide, because one day, he's going to have to be in charge of himself in this realm.
(Reading through the other answers, I feel compelled to add that our 'media time' is pretty much all screen time, period. That said, I don't live chained to an expectation. Last week I had kidney stones and needed to sleep during the day. Some days, Kiddo read for that time, some days, Scooby Doo was a great babysitter. I try to model healthy attitudes toward media/screen time as well. It seems to work for everyone not to have rules set in stone. And like Mamazita's kiddo, mine also enjoys learning about technology and coding. Think hard about what you are *wanting to happen* (as opposed to fearful thinking/action) as you change things before adopting new rules for your girl.)