Too long. My eyes don't tend to hurt until about 2-3 hours in if I don't watch the time. Either way, f.lux helps a lot at night, which is when I'm usually on during the hols. It adjusts the screen so it's more natural, if that makes any sense?? it helps a lot with eye strain. Link to their website - [link] ((oh man i'm a walking advert)
I've put some time and money into having a properly ergonomic setup: my monitor is positioned so that its top is about level with my eyes, my keyboard and tablet are situated such that they're at around the height of my elbows when my arms are dangling by my sides.
If I spend most of a day sitting in a chair reading stuff off the iPad, I get a headache from angling my head down to read it.
Check your glasses, they might need a new prescription.
Also make sure the room is adequately lit. Open windows in the day for some sunlight, turn on some lights at night so you're not staring into a glaring monitor that's the only source of light.
I can spend 3 hours on my tablet. But if i don't have my tablet usually 1 hour. Professionals don't exactly spend all day painting. Well, some do. But it usually depends on the complexity of what they're drawing and usually how quick they can paint and stuff. Sometimes i take 10-20 minutes just fixing up my lineart(which makes me angry). But the pros that I see usually only spend 2-3 hours. And I'm pretty sure that staring at a screen 10 hours a day will be very unhealthy. So you should lessen your time on the computer. And artists(well, everyone) can't just adjust over time; it can actually worsen overtime sometimes. Last night I had the BIGGEST migraine and i swear i felt like dying. So I'm spending less time around technology for a while until i get better.