I always order by the glass because my better half doesn't really drink.

It's annoying because wine by the glass has usually been open for a while and is noticeably oxidized frequently. The exception is at a restaurant that takes care of its wine and has invested in the special wine fridges that add inert gas to the bottle as it's emptied. Also, wine by the glass is usually more expensive per unit volume than wine by the bottle.

I had a half bottle the other day; a friend's mom was visiting from Spain and she and I were the only winos at the table. It was a nice amount: about 1.5 glasses each.