Most games I've played as support, things go go that far..Game won't be that long.. I always keep a slot open for wards. Late game ( if the game is like 72 mins or something) I tend to sell things like Aegis for Frozen Heart... .
Hopefully with the new items coming out we wouldn't have to worry about things like this!
Oh and usually there is always 1 person with a lantern. so that helps a tad~
Supports should always ward no matter what...Everyone should be warding there lanes early on, but later on Supports should be doing the major warding. ( dragon, baron, buffs, etc ) Of course a fed team-mate can help ward 1-2.