It's a very personal thing. Some people happily do it all day, others declare it a blurry mess. I'm somewhere in between. I enjoy doing stints of 1-2 hours in there just for a change of scenery and to help me focus. It's perfectly usable for me, but it's not as good as a real multimonitor setup in general.

A key element is that although the resolution is lower than a real monitor, you get to make the monitor bigger. So the limit ends up being when you start having to turn your head to see what's on the other side of the monitor, which sounds benign but is an ergonomic disaster.