Istalling a fixed window in a cavity wall

I work as a handyman, but most of my work is joinery, kitchens etc. Have been asked by customer to put a small window in cavity wall, he says keep it long & narrow and it wont need a lintel! I'm a bit sceptical. I've put internal lintels in before for doorways, but dont know if what he says is true ( about one & a half to two bricks wide). Its location is upstairs, only a couple of courses down from the ceiling (though he would be happy enough if it was higher).
Any advice would be much appreciated, particularly any theoretical maximum width that would be safe without a lintel
Bob

hi

This is all dependant on how strong the masonry is, i agree with gadget upto about 350mm you should be ok (with an opener) but over this length then i would be very sceptical if an opener was fitted if it is a fixed window with no openers then upto 450mm but no more than that