How long will a wedding cake made with buttercream be ok out in the heat?
It will be Crisco buttercream and it's in the Florida heat.

Depends. Will it be all shortening or shortening and butter. What time of the day? Will it be shaded or in direct sun light. I made my cousins wedding cake last summer all butter cream but her wedding was in the evening and the cake table was under a huge shade tree. I personally won't use Crisco I would use Hi ratio if you can get it. I usually limit my cakes to four hours but they are never in direct sun light and you would want to use a filling that doesn't require refrigeration.

Mine holds up fine. 99.9% guaranteed. It uses regular crisco.

Here's the recipe: http://cakecentral.com/recipes/6992/indydebis-crisco-based-buttercream-icing

Here's the outdoor test a fellow CC'er did:
http://cakecentral.com/cake-decorating-ftopicp-6873024.html#6873024

I personally have had a cake sit outside for 5 hours, in 90+ degree heat with something like 90% humidity (the thunderstorm broke out about 15 minutes after I left the wedding, after cutting and serving the cake at 7:00 p.m.) I set the cake up at 2:00 in the afternoon, the hottest part of the day. No melting what so ever.

I made a 2 tier birthday cake and several cupcakes today. we had 101 degree weather in Texas and the event was for a pool party. It held up excellent! I used Shugarshacks buttercream and made sure my cake had enough supports.

Ive used INDYS RECIPE, Which I love, get it requested and recently also Sugar shacks for an out door party since its non persihable, but IVE NEVER had problems with Indys at all and I live in the hot/humid Texas heat!