I'm considering a very similar thing (alcove next to a chimney breast). I reckon 60cm is probably a minimum depth, not just in terms of not being right on your monitor but also to be able to stretch your legs.
Thank goodness you mentioned that. I didn't even consider the leg depth...
I think mines a little more awkward as the window/ door is part of that alcove too, but I'm determined to make this work as I love the light that comes in there.
I'm considering a very similar thing (alcove next to a chimney breast). I reckon 60cm is probably a minimum depth, not just in terms of not being right on your monitor but also to be able to stretch your legs.